Alarm Substances and Alarm Behavior in Social Insects1

Alarm Substances and Alarm Behavior in Social Insects* U. W. MASCHWITZ Institute of Organic Chemistry, University of Heidelberg, Heidelberg, Germany

I. Introduction . . . . . . . . . . . . . . 11. Alarm in the Honey Bec . . . . . . . . . . . A. The Process of Alarming, Localization, and Chrrnical Coniposition the Alarm Substances . . . . . . . . . . . B. The Reactions of the Alarmed Worker Bees . . . . . . 111. Comparative Investigations of Alarm in Social Hymenoptera . . A. Distribution, Localization, Chemical Composition, and Discharge Alarm Substances . . . . . . . . . . . . B. The Behavior of Alarmed Hymenoptera . . . . . . C. Quantitative Investigations of Alarm Systcins . . . . . D. Specificity of Alarm Substances . . . . . . . . IV. Alarm in Termites . . . . . . . . . . . . . V. The Evolution of Chemical Alarm as Exrmplifird in Hymenoptera . VI. Concluding Rcinarks . . . . . . . . . . . . Rcfrrences . . . . . . . . . . . . . .

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269 271 275 275 279 282 283 285 286 288 289

I. INTRODUCTION Organisms living together in organized groups must be able to communicate in order to make tlic group a harmonious unity. This applies both to the single cells from which a higher organism is built u p and to thc individual members of an animal community. The most highly developed animal societies can be found among social insects. These dispose of a multitude of different ways of communication for regulating their social living requirements, such as the obtaining of food, care of offspring, search for a domicile, and defense against enemies. The signals used thereby can be mechanical, optical, or chemical in type. Here we will consider alarm in social insects, i.e., their ability to inform their nest inatcs of an enemy attack. The present review deals almost exclusively with social Hymmoptera because only few investigations have been performed on the danger alarm of the equally social living termites. As research ovcr tlie last 10 years tias shown, communication concerning danger among insects is brought about predominantly by chemical means. Other groups of aniiiials, such as fish (von Frisch, 1941 ; Schutz, 1956; Pfeiffer, 1960, 1963n, 1963b), frogs (von Eibl-Eibesfeldt, 'This article was translated from the German by Scripta Technica, Inc. 267

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U. W. MASCHWITZ

1949, 1962), and snails (Kempendorff, 1942), also possess chemical alarm systems. They therefore will be referred to herein for purposes of comparison. Alarm signaling by means of odor substances was discovered many years ago. Huber (1814) found that the freshly extracted sting of a worker bee excited other workers and made them aggressive. However, his observation received little attention. Subsequent reporters have either entirely overlooked the problem or have assumed the existence of optical or mechanical alarm systems similar to those of humans, rather than chemical ones. Goetsch, who in 1934 noted chemical alarm in ants, did not follow up his observations. The real credit for recognition of this mode of communication must go to von Frisch, and his are the earliest investigations. I n 1941 he published his classical investigation on alarm substances in the European minnow (Phoxinw laevis Ag.), which is a small freshwater fish living in groups. I n social insects, this problem was studied for the first time by Sudd (1957), Wilson (1958), and Butenandt e t a2. (1959). Alarm substances are to be classified among the pheromones (Karlson and Liischer, 1959). “Pheromones” are substances that are produced in exocrine glands and evoke reactions in members of the appropriate species.la This expression which was coined in analogy to the term “hormone” overlaps widely with the term “releaser” used in ethology. Sexual attractants, stimulants, etc. represent chemical signaling devices which do not basically differ from optical or mechanical releasers [for a discussion see also Mark1 and Lindauer (1965) and Schultze-Westrum (1965)l. No definite proof of a direct effect of such substances on the metabolism of a member of the species in the sense of a hormonal effect has yet been demonstrated. According to existing knowledge, an alarm substance can be defined as a glandular substance which releases specific alarm reactions in conspecifics, e.g., attraction or flight. A further criterion in insects is that they can store the alarm secretion in a glandular reservoir and spontaneously eject it on excitation. Accordingly, attractional secretions that are not discharged during threat of danger (e.g., queen substances or trail substances in some ants) do not come into this category. Fishes, snails, and tadpoles cannot on excitation, in contradistinction to the foregoing, actively give off their alarm substances, which are set free only when the animal is injured. ‘“A definition for active substances which do not merely act intraspecifically was given in 1958 by Kirschenblatt (see Kirschenblatt, 1962). Substances which exert effects between individuals of the same species (corresponding to pheromones) he designated aa homotekrgones. Heterotelergones embrace all substances that are excreted by members of a species and act on other species, e.g., poisons, repellents.

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The alarm system of the honey bee has been thoroughly studied by various investigators (Ghent and Gary, 1962; Boch et al., 1962; Maschwitz, 1964b; Shearer and Boch, 1965). This will be described and subsequently will be compared with those of other social insects. 11. ALARMIN

THE

HONEYBEE

A. THEPROCESS OF ALARMING, LOCATION, A N D CHEMICAL COMPOSITION OF THE

ALARMSUBSTANCES

The typical course of an alarm will be first described. It can best be demonstrated by shortly touching with a small forceps the few workers

FIG.1. Worker bee in a state of alarm, after having been touched. The sting chamber is open, the sting protruding ; and the hairy membrane, between the bristles of which the alarm substance is openly stored, is everted (arrow). The bee is also beating its wings (alarm fanning). Reproduced from Maschwitz (1964b).

that sit in front of the entry to the beehive on a cool day. The irritated bees promptly rise their abdomina steeply in the air, open their sting chambers, and protrude their sting apparatuses (Fig. 1). I n this “fanning” attitude the workers with wings whirring, run into the hive, from which immediately emerges a score of excited bees, ready to attack any opponent. This attraction of worker bees is caused by a chemical signal. It can be shown by the following experiments: If one presents a t the entry to the hive a freshly isolated sting apparatus having its typical

270 U. W. MASCHWITZ

FIG.2. (A). Attraction effect of the sting alarm substance on worker bees (test reaction). .4 freshly extracted sting apparatus on a fixed filter paper is presented on the flight board of a bee hive. Xumeroils worker bees are seen to run about and examine the smelling paper. (B) Control test with a blank filter paper; the latter is ignored. Reproduced from Maschwitz (196410).

Y

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aromatic odor or an ethereal extract of such a sting, the same alarm effect is produced. On the other hand, when bees not having a sting apparatus are strongly irritated and allowed to hurry into the interior of the hive, they do not trigger any alarm. The whirring of the wings is thus not of ibelf an alarm signal. It is the perfume which not only excites, but also attracts, the bees; they often run to and fro and examine the source of the scent. This attractive effect can be used as a test for the invest.igation of the danger alarm substance, e.g., for the location of the odor substance in the sting. For this purpose one presents to the workers squashed glands and parts of stings on a piece of filter paper and counts the number of bees touching the paper; blank papers are provided as controls (Fig. 2). The sting apparatus consists of a complex system of chitinous parts, musculature, and glands (Fig. 3 ) . A set of proximally located chitinous plates moves the distal hollow sting instrument, the aculeus. A densely bristled chitinous membrane extends between two of the plates. The test showed that the alarm substance occurs free between these bristles. The sting apparatus has so far been found to contain four glands. Whether one of these produces the alarm substance has not yet been decided unequivocally. Three glands can be certainly ruled out as production sources. The poison gland and Dufour’s auxiliary sting gland open directly into the aculeus, so that the alarm substance cannot possibly reach the designated place. The paired sting sheath gland can also be eliminated, as shown by extirpation experiments. Ghent and Gary (1962) have suggested that the alarm substance is produced in the fourth pair of glands, known as Koshevnikov’s gland. However, tests with these glands were negative (Maschwitz, 1964b). If during the described alarm behavior the bee opens the sting chamber and extends the sting apparatus, the reservoir of the alarm substance, known as the hairy membrane, is everted outward (Fig. 1, arrow). I n this way the readily volatile alarm substance is set free, without the bee being obliged to sting. The alarm substance is also emitted during the act of stinging, in which case the site of the sting is rendered odoriferous, especially if the sting remains attached to the opponent, I n addition to the sting alarm substance the worker bee possesses a further aromatic substance which sets off an admittedly much weaker alarm, namely the secretion of the mandibular gland (Maschwitz, 1964b). If the bee is strongly stimulated mechanically, it emits this secretion as well. Only the worker bees possess danger alarm substances. Male bees have no alarm secretions. They possess no poison sting apparatus, since the latter is derived from an egg laying apparatus. The mandibular gland of the

ALARM SUBSTANCES AND BEHAVIOR IN SOCIAL INSECTS

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drones is rudimentary. Likewise the queen bee has no alarm substance. Its sting apparatus is more primitive than that of the workers and has no bristle-equipped membrane. Her mandibular gland secretion has other functions.2

PS

-

FIG.3. Sting of a worker bee (modified after Zander, 1951; Snodgrass, 1956; Oeser, 1961; Maschwitz, 196th). b , bulb of the stylet; D g l , Dufour’s gland; fr, furcula; ipo, incisura postarticularis; K g l , Koshevnikov’s gland; Zcl, lancet of the sting; ob p , oblong plate; paa, pars articularis; pgl, poison gland; p m , processus articularis; ps, poison sac; qu p , quadrate plate; rl, rz=first and second basal rami; sm, hairy membrane (alarm substance reservoir) ; s t sh, sheath of the sting (slmith gland) ; f r p, triangular plate.

Tlic chemical nature of two of the bee alarm substances has been elucidated in reccnt years. Boch et al. (1962) identified a main component of the sting alarm secretion as isoamyl acetate. This has a potent attractive effect, but does not evoke typical aggressive behavior. The *Both in the mandibular gland and in a gland of the sting apparatus, i.e., Koshevnikov’s gland, the queen produces substances that are of social significance and serve inter alia for regulation of caste (Butler, 1954; Butler and Patton, 1962).

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authors therefore assume that there is another active substance in this alarm secretion. The aromatic substance of the mandibular gland of the worker bee was identified by Shearer and Boch (1965) as 2-heptanone.

B. THEREACTIONS OF THE ALARMED WORKERBEES The reactions of bees excited by the sting alarm factor will now be considered in greater detail. No corresponding investigations have so far been performed on the mandibular gland ~ e c r e t i o nThe . ~ alarm substance which has been set free by alarm fanning or stinging excites other worker bees in or near the nest, as evidenced by patrol running or flying and readiness for attack. Since the alarm secretion is also attractive it guides excited bees t o the source of the odor. Therefore the secretion is of great importance not only in alerting the bees, but also in the localization of the enemy. The attack itself is released by stimuli coming from the enemy. I n the dark hive these are strange scents and tactile stimuli. Additional optical signals are necessary when the bees attack an enemy in front of the hive. If one presents to excited bees a n alarm substance on a fixed white surface that is unattractive in itself, they do not attack it. Factors most inciting aggression are the movement of an object and, to a less extent, a dark color. Besides this, the size of the object affects the aggressiveness of bees: a three-dimensional, dark-colored object, 610 cm in diameter, is optimally efficient in releasing attack. If an optically attractive object is additionally treated with the alarm substance, the bees attack it more eagerly. For example, in a series of experiments the bees visited a spherical object scented with alarm substance ten times as frequently as a n unscented one. Objects scented with alarm substance are stung much more frcquently and severely than unscented ones. The intensity of stinging by attacking bees is further heightened by velvety surfaces (Lecomte, 1961) and by increased temperature of the object. I n summary i t can be said that the alarm secretion only attracts and excites the worker bees, whereas attack is triggered by other stimuli. The stimulus pattern combining the attack-triggering properties outlined (moving, dark-spotted, three-dimensional, warm, velvety surface) is best met by a niedium-sized speckled r n a m n ~ a l .The ~ ~ warm humid breath of a mammal also incites bees to attack. This is the only stimulus, other than mechanical, that can incite the worker bees to alarm Sinipson (1966) found that the mandihlar gland srcrction has a repellent effect on foraging bees. ’” Not only the rrleasing stimuli for attack brliavior in the worker bee, but also the morphological peculiarities of its sting apparatus which makr sting autonomy possible, show that the attack reaction of the worker honey bees is directed primarily against mammals or other large vertebrate animals (Rietschel, 1937).

ALARM SUBSTANCES AND BEHAVIOR IN SOCIAL INSECTS

275

fanning. Thcrr is no evidcncch that rlicwiicnlly al:irmcd l m s theniselves respond by giving off an alarm sccrction. Tlie worker hccs react to the alaim hccrction in an entirely different manner when they are outside their yocia1 community, e.g., a t tlie feeding place. Under such circumstances some iiiild excitation reactions can be observed, but no attack reactions arc triggered. Tlie bees often become rcstless i n thc prraence of tliv aliiriii substance, and sometimes even fly away. 111. COMPARATIVE: IN

INVESTIGATIONS O F

ALARM

SOCIAL HE'MEKOPTERA

A. DISTRIBUTION, LOCALIZ.4TION, OF ALARMSVBSTANCES

CHEMICAL COMPOSITION, AND

DISCHARGE

Like tlie honey bee, most ohther social Hymenoptern possess alarm substances. This was demonstrated by tests with isolated glands. Our knowledge of tlie distribution of the chemical alarm signaling in this animal group, as Table I shows, still is rather incomplete. Tlie alarm substances in tlie various social Hynicnoptcra are produced in different glands and stored in reservoirs from which they can be emitted when the insects are irritated. Table 11 shows, in typical examplcs of each family arid subf:tmily of social Aculenta, the glands that produce alarm substances together with all possible comhinations so far discovered (Fig. 4 ) . Also included are the alarm substances the chemical structure of which is known. Tahle I1 also shows the species that do not produce any alarm substance. Table I1 demonstrates that in all three families species exist which do not possess any chemical alarm mechanism. Such species are socially more simply organized. It is further seen that not only tlie honey bee but also many ants possess several glands with an alarm function, e.g., Myrmicinae and Camponotinae. For such multiple provision there are a number of possible interpretations. For example, in Myrniicinae the secretion of the poison or accessory gland has a different significance according to the circumstances of emission (demonstrated in T'etran~oriu?nand Solenopsis). Set free when attacking, it attracts the nest mates which can engage in tlie overpowering of the enemy. If, on the other hand, a worker finds a rich food source it marks a scent trail with its sting lcading to the nest, which brings other ants t o the feeding place. Tlie substance thus in both cases calls workers to the place where they :we needed. This feature does not, however, pertain to Formica which does not lay any scent trail. Since the glandular substances are exhaustible, the possession of multiple alarm releasers also in this ant is advan-

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U. W. MASCHWITZ

tngcous. A phylogenetic intrrprettttion of this niultiple provision is given

in Section V. Table I1 shows furtlierniorc. that t,he sources of alarm substances are morphologically closely assoriated with thc defense organs and defense glands, e.g., mandibles, sting glands, and anal glands. I n some types the abdominal defense glands are actually identical with the glands that produce alarm substances-e.g., in Vespidae: Vespa; Formicidae: Formic,a, Myrmica, and Tapinoma. I n other cases in which defense glands and alarm substance-producing glands are not identical, the secretions are nevertheless emitted simultaneously and so are mixed, e.g., Lasius and Apis. The close relation between defensive and alarm secretions is also manifest in tlicir chemical composition, as shown in

a:

FIG.4. Schematic figure of a worker ant (Dolichoderinae), in the body of which all glands and organs used by social Hymenoptera for alarm signaling and trail laying are represented. The following glands in Hymenoptera contain alarm substances: MG, mandibular gland; PG, poison gland; and DG, Dufour’s gland. The actual alarm substance gland of Dolichoderinae is the anal gland ( A G ) which is absent in all other Hymenoptera. The following organs producing or storing substances used solely for trail laying are: h gu, hind gut; and sg, sternal gland of Dolichoderinae = Pavan’s gland. m, mandible; s t , sting; r, rectum. (Gastric glands adapted from Wilson, 1965.)

Table 11. There are cases where a single substance simultaneously has a poison and an alarm effect, as exemplified by formic acid in Formica or ketones in Dolichoderinae. I n Vespa and various Myrmicinae, the volatile alarm substance is mixed with the poison which is a complex mixture of proteins (Maschwitz, 1964b; Blum and Ross, 1965). Apart from the formic acid with its simple chemical structure the known alarm substances are terpenes, ketones, and esters. Our knowledge in this field is regrettably incomplete. Only for seven of the several thousands of species of Hymenoptera possessing likely chemical alarm systems is the chemical nature of alarm substance known.4 ‘In various species of Dolichoderinae, Cavill and Hinterberger (1961) found, inter aliu, ketones, which are supposed to act as alarm substances.

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ALARM SUBSTANCES AND BEHAVIOR IN SOCIAL INSECTS

TABLE I NtlMBEH OP G E N E R A ANI) SI'ECIKS OF SOCIAL IIYMENOPTEHA S O l 7 A H [(NOWN, AND

Family Subfamily

Alarm substance"

Apidae Apinae Meliponinae Rombinae

+

Formicidae Myrmicinae Campono tinae Ponerinae & Myrmiciinae Dolichoderinae Dorylinae Other subfamilies Vespidae Vespinae Polistinae Other subfamilies a

Symbols:

NUMBERINVESTIGATED FOR ALARM SUBSTANCES Social genera

Sorial species

Number Number knownb investigate&

Number Number known investigated

? -

3 2

0 1

4 266 200

1 0 3

* *

126 115 52

13 4 4

2500 2200 900

22 9 4

?

17 6 14

5 3 0

300 200 250

6 3 0

1 3

1 1 0

550

2 1 0

+

+ + + ?

1

1

?

+, alarm substance present; -,

1

no alarm substance found; ?, no data;

*, alarm substance present in some, absent in others.

Apidae and Formicidae after Berland and Bernard (1951); Vespidae after Peters (1965). c Summarized from Wilson and Bossert (1963), Wilson (1965), Maschwits (1964&, and unpublished observations).

Since the poison and the abdominal alarm secretions are either identical or are always emitted together during fight, emission of poison always implies simultaneous operation of an alarm mechanism. I n fighting with mandibles, alarm substances are emitted from the mandibular gland; this has been observed, for example, in Myrmicu, Dorylinae, and the honey bee. However, it should not be concluded that alarm signaling is necessarily always a simple side effect of fighting behavior. Social Hymenoptera can emit their alarm secretion without biting, stinging, or spraying their opponents with poison. If, for example, one irritates Myrmicu workers by holding or pinching, they release both mandibular gland substances and alarm signaling poison. Likewise Vespa sprays out its poison when mechanically irritated. Many ants, such as Tupinoma or Lasius, also, like the honey bee, show a typical alarming position. They too raise the posterior part of the body and release a droplet of poison-alarm substance from its tip, and run to and fro among the

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U. W. MASCHWITZ

TABLE I1 GLANDULAR SOURCES A N D CHEMICAL COMPOSITION OF ALARMSUBSTANCES IN SOCIALHYMENOPTERA

Family Subfamily Genus and species Apidae Apinae A p i s mellijicae~f*o

Mandibular gland secretion

+"

Other secretions Poison Dufour of sting gland accessory gland secretion secretion glands

-

-

(2-hep tanonep'g)

Solenopsis saevissimai A phaenogaster testaceag Myrmecina graminicolak Camponotinae Formica polyctenao Lasius nigero Acanthomyops claviger'

Ponerinae Paraponera clavata' Ponera coarctatak Dolichoderinae Tapinoma nigerrimao

Dorylinae Eciton hamaturn" Vespidae Vespinae Vespa vulgaris0 Polistinae Polistes dubiao

+

ob

(isoamylacetate)

Bombinae Bombus lucorumg Formicidae Myrmicinae Atta*,i

Anal gland secretion

0

+

0

+i

(citralh; (A.tezana) A . sexdens) +?c +? -

+ -

+ (citral,

0 0 0

+

(formic acid)

-

0

+?

-

-

0 0

citronellalm)

+-

0

0

+

(methylheptenone, ProPYl isobutyl ketone")

+?

?

?

-

0

ALARM SIJUSTAIVCICS A N D UEHAVIOlt I N SOCIAL INSECTS

279

members of thc colony. The alarm signaling behavior thus has become partially divorced from the fighting behavior and has become an independent instinctive pattern. Thereby the alarming workers are fairly mobilc and can carry the exciteincnt on into the nest. I n the first place the workers emit the alarm substance after direct mechanical stimulation. I n the honey bee humid warm air can, as already mentioned, also release alarm fanning. According to Wilson (1958) high concentrations of alarm substance can very probably cause secondary alarin in the ant Pogonomyrmex badius. This phenomenon has not so far been detected in other Hymenoptera. With respect to the ontogenesis of the alarm signaling reactions and the beginning of production of alarm substances, there have been made only a few observations in thc case of the honey bee. These showed that recently hatched bees exhibit the typical alarm fanning, although they do not yet possess any alarm suhstancc. The secretion develops gradually in the course of the first weck of life as imagos.

B. THEBEHAVIOR OF ALARMED HYMENOPTERA I n contrast to pure flight reactions which the alarm substances of fishes, frog larvnc, and snails rclcasc in conspccics, the behavior of chemically alarmed Hymenoptera can be very different. Above all i t depends on caste and sex. Only workers display the typical alarm reactions. We do not yet know whether the scxuals react a t all. At any rate they do not take part in attack. (Reproductive females, except in the honey bee, posscsn alarm substances as far as has been investigated. I n Vespa the f,

Alarm substance produced;

-,

* 0, Insect lacks gland in question.

no alarm eflect.

+?,Supposed alarm production. ?, Not known whether or not secretion produces alarm effect. Gherit and Gary (1962). f Boch et nl. (1962). Maschwita (196411). Butenandt el rrl. (195!)). ' Moser and Blum (1963). f Wilson (1959). Maschwitz (1964a). Ghent, 1961, after Wilson (1965). Chadma et al. (1962). Trave and Pavan (1956). " Brown (1960). t~ Shearer and Boch (1905). * Also found as alarm substmice in the dolichoderine ant Zridomyrmex pruinosus (Blum et d.,1963). Wilson (1965). c

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U. W. MASCHWITZ

females have been observed to emit them when irritated.) Further, the reactions of the workers are not uniform. They vary not only according to the species, but also with the situation (e.g., in the nest or at a feeding place), population strength, and concentration or duration of exposure to the alarm substance. They may be influenced also by the age and the current activity of the workers. No investigation has so far been performed on ontogenesis of reactivity to alarm sub~tances.~ Alarm reactions occurring in adult workers range from simple attraction to attack or flight. According to Goetsch (1934), thanatosis also occurs. Since there is a lack of detailed studies on the behavior of individual animals in different physiological conditions, only the average behavior of the workers can be described herein.

1. Attraction and Aggression An alarm secretion does not usually in itself release an attack, for otherwise the alarming members of the colony would be killed. Such killing has been observed only in a few extreme cases (Butenandt et al., 1959). Attack-releasing stimuli come from the enemy. They have been investigated systematically only in the case of the honey bee and to a certain extent also in Vespa (Lecomte, 1961; Free, 1961; Maschwitz, 1964b). Upon release of alarm substance in the nest all social Hymenoptera thus far as investigated show attraction behavior. Additional excitation (i.e., lowering of the threshold of the attack reaction) often can be observed. The resulting behavior can be described somewhat as follows: The alarmed workers run to the place of the emission of the alarm substance; they examine i t with their antennae, circle it excitedly, and return several times to the odor source. Ants and wasps frequently open their mandibles, and some of them bite into the scented object and carry i t away. Alarming workers are examined and then left alone by their nest mates. I n the ant Pogon,omyrmex badius, Wilson (1958) states that attraction and aggression are released by different concentrations of the alarm substance. According to this view, amounts of alarm substance just over the threshold level evoke a predominantly attractional behavior, which passes on to marked excitement as the concentration increases. Such dependence of reaction on concentration is apparently widespread. However, other factors too can trigger the increased appearance of either of the two reactions. I n Apis, according to Boch et al. 'It is known that in cyprinids reactivity to an alarm substance develops only after the fish are sevcral weeks old, whereas the substance itself appcnrs rather e:irlicr (Schutz, 1956; Pfeiffrr, 1963a).

ALARM SUBSTANCES .4ND BEHAVIOR IN SOCIAL INSECTS

281

(1962), attraction and exciteiiient are possibly released by different substances (see Section 11, A ) . Workers of Vespa generally only examine a scenting object when it is dark outside the nest, but readily attack in bright daylight. The fact that all Hymenoptera, including even very small and weak ants, seek out in the nest the place scented with the alarm substance seems biologically significant, since they have to protect both the queen and the brood, whose lives are necessary for the existence of the colony. The situation a t the feeding sites, on thc other hand, is quite different. Here, only large ants capable of dcfense, e.g., the mound-constructing species of Formica or Tapinoma nigerrima, show attraction and aggression behavior (Maschwitz, 19f34b). Workers of thcse species, while struggling with a strongly resistant predatory animal, spray i t with poison and thereby attract other ants in the vicinity to the conquest of the prey. They thus possess through their danger alarm substance a rapidly acting chemical prey alarm, which has been seen to have an extremely powerful effect. I n a few seconds Formica can in this way overcome prey that a single worker could not have subdued. This mode of prey alarm among predatory ants may possibly be widespread, e.g., among the blind army ants. 2. Flight Reactions and Other Modes of Behavior

A distinct flight behavior could bc observed a t the feeding place in various species of ants, e.g., in the small Tapimma erraticum. When an alarm substance was placed a t the feeding place, the workers ran back to the nest either immediately or aftcr some time. Wasps, and also honey bees, likewise never attacked a t the feeding place on presentation of alarm substance. They too displayed weak flight reactions, became restless, or soon flew off. Flight reactions a t the feeding place are advantageous to Hymenoptera, which are not specialized hunters. It is not worth while for the colony to sacrifice valuable workers for the constantly changing food sources. I n Tapinoma erraticum one could obscrve in the proximity of the nest conflict reactions between attraction, aggression, and flight. The workers ran excitedly toward the scented spot with wide open mandibles, then stopped, and flew off. Wilson (1958) described in the harvesting ant Pogommyrmex badius a further reaction which could be evoked by presentation of high conwntixtions of niandibular gland secretion over a period of several minrites. Soiiic of t l i c i workers bt1g:tn to dig a t tlie soiirce of the alarrii

282

U. W. MASCHWI'TZ

substance. I n this way alarming workers confined in the nest under natural conditions can be freed.

C. QUANTITATIVE INVESTIGATIONS OF ALARMSYSTEMS Bossert and Wilson (1963) for the first time studied chemical alarm mechanisms and other olfactory modes of communication among animals on a mathematical basis. The natural situation of an alarm in the nest of social Hymenoptera corresponds roughly to the model case analyzed by the authors, in which the substancc in still air is set free in a puff. Their calculations led them to the conclusion that the decisive parameters here were the diffusion constant D and the Q : K ratio, i.e., the ratio of alarm substance molecules released to the threshold concentration per milliliter. These two values can be estimated empirically if one determines the onset times of responses of the experimental animals and the corresponding distances from the source of the scent. Bossert and Wilson carried out such investigations on the a n t Pogonomyrmex badius. The diffusion constant for the alarm substance of the mandibular glands was 0.33-0.88 cm2/sec. On liberation of the total alarm substance of one worker there was a Q : K ratio of 939 - 1800. An approximate estimation of Q and K separately showed that the head of a worker contains about 6.26 X 10l6 molecules of alarm substancc. The maximum sphere of attractiveness of the alarm substance of a worker was about 6 cm, and this was attained in about 13 seconds; in about 35 seconds the signal had disappeared completely. On the basis of these data the authors attempted to determine general conditions for the optimal effectiveness of a chemical alarming system in insects. T o prevent the entire colony from being disturbed by minor local distractions which induce one or a few workers to alarm, i t is important that the alarm effect should wear off quickly and that the alarm should not reach to a great distance. This can be achieved on the one hand through a high diffusion constant resulting from the molecular properties, and on the other hand physiologically through a moderately high value for the stimulation thrcsliold of the alarmed animals. I n the event of stronger disturbance and associated greater danger, the alarm reaches farther and is of longer duration because of the increased number of directly stimulated alarming animals, i.e., increase of the mass of alarm substance set free. When the danger stimuli cease, the signal nevertheless quickly fades. Phylogenetic discussion of this point follows in Section V. Except for thc results of Bossert and Wilson, there are no quantitative studies on r1iciiiir:il alarni systeiiis. Tllanlts to the pioneering work of the authors, this important aspect can be easily investigated in the future.

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L>. SPECIFICITY OF ALARMSUBSTANCES Social Hynienoptera react in a very sensitive manner to foreign odors. Any objects or intruders entering the nest are rccognized by the absence of the colony scent-a mixture of species-specific odors and especially of aromas emanating from the feeding niaterial and the nest material (Renner, 1960; Lange, 1960). A number of sriielling substances introduced into tlie nest arouse excitement among the workers and in extreme cases actually cause thein to lcave tlie nest (Martin, 1963). Therefore, before one can speak of alarm substances, it must be determined whether the glandular substances, as compared with other aroinatic substances, induce truly specific alarm reactions. Some qualitative investigations were niacle to compare tlie action of alarm substances with those of aromatic chemicals and scents of plants and animals. In general, the alarm reaction was specifically caused by tlie alarm substance. I n some cases, however, one could also observe that foreign odors evoked reactions that closely resembled the behavior elicited by the alarin substance, c.g., flight reactions in ants at the feeding place caused by unsaturated aldehydes (Remold, 1962) or attack reactions produced by lower fatty acids in the ant Poyon.omyrmex badius (Wilson, 1958). Vespa, which normally reacts specifically to the alarm substance, is provoked to flight, while in a state of extreme excitation, by other aromatic substances as well (Alaschwitz, unpublished niaterial). In this connection it is noteworthy from the evolutional aspect that in social Hynienoptera (e.g., Polistes) , which do not yet possess any specific alarm-releasing substances, an excitation reaction can be evoked by foreign scents. How far substances cheiiiically similar to the alarm substance (isoniers, liomologs, etc.) can act as alarm releasers is unknown.5" The action of the individual coiiiponents of the natural alarm secretion and their interplay one with another will also have to be studied in greater detail. In this respect much further work is necessary on the chemical nature of the suhstanees involved. Likewise the interesting problem of species and genus specificity of the alarm substances can be elucitlated only by further cheniical work. So far, various authors have been able to show that among the social Hymenoptera related species produce nlariu secretions that are reciprocally effective. For cxaniplc, the alarm substances of four species of Myrnrica (Fig. 5 ) ) five species of Lasz'zls, and two species of Paravespula act intragenerically. To a certain rxtent various genera do possess reciprocally acting alarm substances, as seen in the secretion of Dufour's In the meantime suc~ha study 113s been performed by Blum et al. (1966) with a series of different ketones in Jriclomyrmex pruinosus.

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glands in three genera of Camponotinae and the poison gland secretions of four genera of Attini (Maschwita, 196413; Blum et al., 1964). It is not yet known whether such reciprocal ability to cause alarm is brought about by chemically similar substances or by relatively nonspecific reactions of the workers. Blum and Ross (1965) gave a further possible explanation for this fact. Although various groups of ants possess a number of common aromatic substances in their glands, each species uses one of these as releaser, which is different from those used by other species. Species t e s t e d Hyrnica ruginodir

m

n

m

P

Hyrmicr

laevinodir

P

P

m

P

Hyrmicr

My m i c a

m

m

rulcinodis

P

rubida

P

FIQ.5. Specificity of alarm substances within the ant genus Myrmica (Myrmicinae). Tests with mandibular gland and poison gland secretions. Left column in each group (white) = control test without odorous substances; right column (black) = number of ants which reacted to the alarm substance of a strange species (as percentage of the number of workers which react to the alarm substance of hatched column). Figures above the middle their own species-horizontally columns are absolute values (100%) for their respective columns. Reproduced from Maschwitz (1964b).

On the other hand, the abdominal alarm substances, insofar as they are also used for trail laying, are partly species specific (Wilson, 1959; Blum and Ross, 1965). Invest,igations on species and genus specificity of alarm substances have yielded conclusions about taxonomic interrelationships, as von Frisch (1941) has shown in the case of Cyprinidae (Teleostei). His findings fully confirm the relationships proposed on the basis of morphological characteristics. Similar findings for social insects indicate t h a t the alarm reactions might be used in addition to morphological criteria for characterizing related groups. Even quite delicate differences can be ascertained. For instance, on the basis of the differential alarm effect of the mandibular gland substances within the genus Myrmim, it was possible to confirm the morphological results that Myrmica rub& has to be classified into a special subgenus (Fig. 5 ) . In general, however, one can-

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not draw such far-rcacliing systematic conclusions in the case of the social insects as within the fish group Ostariophysea. Whereas in the latter the alarm reaction has very probably developed monophyletically (Pfeiffer, 1963b), the chemical alarm reaction in the social Hymenoptera has undoubtedly arisen polyphyletically (see Section V) . These interspecies alarm effects probably have on the whole no biological significance in the case of the social insects. The social parasites are an exception. It could be observed that the social parasitic ant Strongylognathus huberi which lives with Tetramorium mespiturn possesses, both in the mandibular glands and in the gaster, alarm substances that have an even stronger excitant action on the host than the latter’s own alarm substance. This signifies that an attacked and alarm-raising social parasite can summon the workers of the host colony to its defense (Maschwitz, unpublished material).

IV. ALARMIN TERMITES I n contrast to the social Hymenoptera, very little is known about alarm in termites. So far, only Stuart (1963) has dealt exhaustively with this problem-in the case of the termite Zootermopsis nevadensis Hagen. According to his findings, alarm in this species is transmitted by direct contact between individual^.^" Highly volatile substances are not used for alarming in this termite. Nevertheless, workers are able to attract nest mates to points of disturbance, e.g., breaches in the nest wall. They use the secretion of their abdominal sternal glands to lay an odor trail to the point of danger, to which other termites are attracted and can assist in repelling the invaders or repairing the nest. It is probable that the trail substances can also be used for other purposes, e.g., for keeping the individuals banded together or for removing them to another nest. In higher termites which forage away from the nest, the trails are also employed to recruit workers for new food sources. The soldiers of the Rhinotermitidae and Termitidae possess a strongly smelling frontal gland secretion with which they spray their enemies. Whether the aromatic substance of such a secretion functions as an alarm substance has not so far been demonstrated with certainty. Ernst (1959), who investigated this spraying in the case of a Nasutitermes species, inferred that danger alarm was thereby given simultaneously. The same was presumed by Moore (1964) , who succeeded in identifying the aromatic substances of NasutiteTmes as pinenes. However, according to the preliminary inGb Howsr (1965) has found that strongly disturbed workers of Zootermopsis angusticollis perform oscillating mowmrnts when they touch nestmates. These movements relrase rxcitrment in the toriched animals which will then normally follow an odor trail.

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vestigations of Mauchwitz (unpub1ishc.d material), a specific alarm effect of secretions produced by Nnsitfitcrmes ephrntae Wolmgr. is doubtful."

V. THEEVOLUTION OF CHEMICAL ALARMAs EXEMPLIFIED IN HYMENOPTERA I n the case of Hymenoptera some statements can be made about the evolution of the chemical alaim, although our knowledge about the distribution of these types of communication, the composition of the secretions, their adaption to the dcinands of an alarm system, and the ontogenesis and physiology of alarm substances and alarm reactions is still incomplete. Species of Hymenoptera which are on a relatively low level of social development and form only small colonies, such as Polistes (Vespidae) , Bombus (Apidae), and Ponera and Myrmecina (Formicidae), still do not have any chemical alarm mechanism (Table 11). I n these communities this does not appear t o be necessary, since all the inhabitants of small nests can detect any disturbance directly.' Chemical alarm signaling occurs only in highly developed social Hymenoptera. It must therefore have developed polyphyletically within the different families that go back to solitary ancestors; ants, wasps, and bees have developed this mode of communication independently of one another. Within the family of ants chemical alarming may also have developed polyphyletically, since primitive ant genera of different subfamilies are not able to alarm and since the alarni substances producing glands are in part not homologous (Table 11). There are also theories about the phylogenetic development of chemical alarm systems. It is hardly necessary to discuss the necessity and the biological advantages of alarm systems in large colonies of Hymenoptera. As discussed below, the development especially of chemical alarming offers both from the behavioral and from the chemical point of view. Whether this represents the sole possibility of alarming is still largely unknown. We are only beginning to learn about the function of another means of communication in ants: sound production by stridulation (Markl, 1965). The behavioral basis for the development of a chemical alarm system could be the previously mentioned nonspecific reaction of excitation of the social Hymenoptera elicited by smelling substances (in the case of ' 1 wish to express my gratitude to Prof. Dr. H. Becker (Berlin) for permitting me to experiment with his termite rolonies. 'Whether the mode of communication in question can subsequently be lost again, as is known to occur among fishes (Pfeiffer, 1963b), cannot be stated in the present state of our knowledge. However, such losses can be anticipated among social parasites.

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Polistes dul)ia, for c~x:~iii~~Ii~, l)y h q i i : i d i ( v t Iwtly 1):Lrth of workers of tlic m i i e species) . Ah c+hcwiicltlI):ihih thr aroiiiatic su1)stltnrcs could be regarded which are regularly given off in tlic event of defensive activity, such as sting gland substances or anal gland substances, or which a t least are closely associated with the defense weapons, e.g., mandibular gland secretions. Odoriferous substances are widely distributed ainong nonsocial Hynienoptera, so that they did not have to be developed anew for the alarming mechanism. For example, Kullenberg (1956) established that not only the highly developed Aculeata but also the Terebrantia possess aromatic substances. The chemical coinposition and the function of most of these substances have been elucidated only in the ease of a few species. A defensive function of the mandibular gland substance, without alarm function, occurs for example in the workers of Lasius fuliginosus (Pavan, 1961 ; Maschwitz, 1964b). Mandibular gland secretions are frequently utilized not only for alarming, but also for other modes of communication (Iindauer and Kerr, 1958; Haas, 1948; Gary, 1962; Pain and Ruttner, 1963; Holldobler and Maschwitz, 1965; Butler, 1954; Butler and Patton, 1962; Pain, 1961). The primary function of the mandibular gland of Hymenoptera remains to be elucidated. The significance of the sting accessory glands (e.g., Dufour’s gland) in nonsocial Hymenoptera is equally obscure. On the other hand, we know the primary function of the main gland of the sting apparatus and of the newly developed anal gland of the Dolichoderinae. They serve as defense and attack glands. The poison glands of Hymenoptera generally contain complex venoms which chemically are proteins (Blum and Ross, 1965). I n the highly developed ant subfamilies of the Caniponotinae and Dolichoderinae, there are, among other substances, odoriferous poisons, e.g., ketones and formic acid. I n the Myrmicinae and in Vespa, aromatic substances are mixed with the protein venoms. In poisonous secretions the taking-over of an alarm function can be well observed. Formic acid, which is the defense secretion of the Camponotinae, has no alarm effect in Lasius and Plagiolepis. Such an action is present in the more aggressive Fornaica (for other multiple functions see Section 111, A). These examples present a model for understanding what has often been regarded as an inexplicable change of function. “Change of function” of an organ can thus be first an extension of function, to be followed later by the loss of the original function or some further change. In this respect, the multiple provisions for alarming, 3s seen for instance in Apis, become understandable. The much weaker alarm effect of the mandibular gland secretion as compared with the sting alarm substances could, therefore be regarded as an alarm function

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in the process of degradation. A ncw mow cfficient sting gland is taking over its function. Further, Bossert and Wilson (1963) have presented contributions to the evolution of alarm substances. Their quantitative investigations have yielded theoretical data on optimally effective alarm systems. In short, an alarm effect that can be precisely graduated both spatially and temporally is best achieved if the aromatic substance is highly volatile and therefore possesses a high rate of diffusion and if a t the same time the Q : K ratio (see Section 111, C) is moderately large; this can best be attained by a moderately high threshold of stimulation associated with a storing up of the alarm substance that allows variation in its dosage. Bossert and Wilson suppose that in the course of evolution the values D and Q : K have progressively approached an optimal value. Substances having a molecular weight of 100-200 would be most effective as alarm substances. This molecular weight assures on the one hand a sufficiently high volatility, and on the other hand an optimal stimulative efficiency for the olfactory receptors (according to Cook in Dethier, cited by Bossert and Wilson, 1963). As far as our meager knowledge of the chemical composition of alarm substances allows us to state, these postulates appear to be correct. The upper limit of the molecular weight of 200 has not so far been exceeded by any alarm substance, and a figure below the lower limit of 100 occurs only in the case of formic acid in the species Formica. This exception requires more careful examination. Formic acid is the highly potent defense substance of the Camponotinae. An evolutionary change in this substance toward optimal properties for an alarm substance would probably cause loss of its poisonous properties, i.e., the two evolutionary tendencies would counteract one another. Hence substances with dual functions whose respective optimal activities diverge widely are unfavorable. Formic acid is therefore a relatively unfavorable alarm substance, and this probably explains its rather infrequent use for this purpose in Camponotinae. Information is not yet available as to whether the same effect must be taken into account in the case of other alarm agents. From the evolutionary point of view, there are several possibilities for obviating this “conflict.” (1) Only glandular secretions which are suitable both for a poison and an alarm substance are used for alarming in addition to their previous function, e.g., the anal gland ketones of the Dolichoderinae. (2) The alarm function is taken over by a secretion which possesses several groups of substances which can evolve independently of one another. This possibility is apparently being exploited with greater frequency, e.g., in the poison glands of Vespu and of Myrmicinae. (3) A gland which takes over an alarm function specializes in this task. How far this possibility is realized in practice is not as yet known

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VI. CONCLUDING REMARKS

A review has been given of an important sector of the field of chemical communication in social insects. It demonstrates the great complexity of the process of alarm signaling by means of odor substances. At the same time the paucity of our present knowledge in this field is evident. The social insects have become one of the most successful types of terrestrial creatures on account of their highly developed social structure. Many of them, especially termites and numerous ants, are serious competitors of human beings. The relatively crude and frequently unphysiological measures taken to combat social insects are often ineffectual. If, however, it becomes possible to understand and imitate their language, man will have a weapon enabling him to “confound them with their own words” and thus to hold them in check. There is no doubt that this feat, quite apart from the purely scientific interest connected with the problem of communication, is a weighty incentive for continuing research in this field. ACKNOWLEDGMENT The author wishes to thank Springer Verlag (Heidelberg) for permission to reproduce the illustrations of Figs. 1, 2, and 5. REFERENCES Berland, C., and Bernard, F. 1951. In “Trait6 de Zoologie” (P. P. Grasd, ed.), Vol. X/II. Masson, Paris. Blum, M. S., and Ross, G. N. 1965. J. Insect Physiol. 11, 857. Blum, M. S., Warter, S. L., Monroe, R. S., and Chidester, J. C. 1963. J. Insect Physiol. 9, 881. Blum, M. S., Moser, J. C., and Cordero, A. D. 1964. Psyche 71, 28. Blum, M.S., Warter, S. I,., and Traynham, J. G. 1966. J. Insect Physiol. 12, 419. Boch, R., Shearer, D. A., and Stone, B. C. 1962. Nature 195, 1018. Bossert, W. H., and Wilson, E. 0. 1963. J. Theoret. BioZ. 5, 443. Brown, L. W. 1960. Psyche 66, 25. Butenandt, A., Linsen, B., and Lindauer, M. 1959. Arch. Anat. Microscop. Morphol. Exptl. 48, 13. Butler, C. G. 1954. Trans. Roy. Entomol. SOC.London 105, 11. Butler, C. G., and Patton, P. N. 1962. Proc. Roy. Entomol. SOC.London Am, 114. Cavil], G. W., and Hinterberger, H. 1961. Proc. 11th Intern. Congr. Entomol. Vienna, 1960, Vol. 3, p. 53. Reisser, Vienna. Chadma, M. S., Eisner, T., Monroe, A., and Meinwald, J. 1962. J. Insect Physiol. 8, 175. Ernst, E. 1959. Rev. Suisse Zool. 66, 289. Free, J. 1961. Animal Behav. 9, 193. Gary, N. E. 1962. Science 136, 773. Chrnt, lt, L. 1961. Ph.D. thesis. Cornell University, Ithaca, New York. Client, R. L., and Gary, N. E. 1962. Psyche 69, 1. Goetsch, W. 1934. Z. Morphol. Oekol. Tiere 28, 319.

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Haas, A. 1948. 2. Vergleich. Physiol. 31, 281. Holldobler, B., and Maschwitz, U. 1965. 2. vergleich. Physiol. 50, 551. Howse, P. E. 1965.Insectes Sociauz 12,335. Huber, F. 1814. “Nouvelles observations sur les abeilles.” Paris. Karlson, P., and Luscher, M. 1959. Nature 183, 55. Kempendorff, W. 1942. Arch. Molluskenk. 74, 1. Kirschenblatt, J. 1962. Nature 195, 918. Kullenberg, G. 1956. 2001.Bidrag Uppsala 31, 253. Lange, R. 1960. 2. Tierpsychol. 17, 389. Lecomte, J. 1961. Ann. Abeille 4, 165. Lindauer, M., and Kerr, W. E. 1958. 2. Vergleich. Physiol. 41, 405. Markl, H.1965. Science 149, 1392. Markl, H.,and Lindauer, M. 1965. I n “The Physiology of Insecta” (M. Rockstein, ed.), Vol. 11, p. 1. Academic Press, New York. Martin, P. 1963. Insectes Sociauz 10, 14. Maschwitz, U. 1964a. Nature 204, 324. Maschwitz, U. 196413. 2. Vergleich. Physiol. 47, 596. Moore, B. P. 1964. J. Insect Physiol. 10, 371. Moser, J. C.,and Blum, M. S. 1963. Science 140, 1228. Oeser, R. 1961. M i t t . Zool. Museum Berlin 37, 3. Pain, J. 1961. Ann. Abeille 4, 73. Pain, J., and Ruttner, M. 1963. Compt. Rend. 256, 512. Pavan, M. 1961. A t t i Accad. Nazl. Ital. Entomol. Rend. 8, 228. Peters, D. S. 1965. Personal communication. Pfeiffer, W. 1960. Z.Vergleich. Physiol. 43, 578. Pfeiffer, W. 1963a. Can. J . 2001.41, 69. Pfeiffer, W. 1963b. Erperientia 19, 113. Remold, H.1962. 2. Vergleich. Physiol. 45, 636. Renner, M. 1960. 2. Vergleich. Physiol. 43, 441. Rietschel, P. 1937. 2. Morphol. Oekol. Tiere 33, 313. Schultze-Westrum, T. 1965. 2. Vergleich. Physiol. 50, 151. Schutz, F. 1956. 2. Vergleich. Physiol. 38, 84. Shearer, D.A,, and Boch, R. 1965. Nature 206, 530. Simpson, J. 1966. Nature 209, 531. Snodgrass, R. E. 1956. “Anatomy of the honeybce.” Comstock, New York. Stuart, A. M. 1963. Physiol. Zool. 36, 85. Sudd, J. H. 1957. Nature 179, 413. Trave, R., and Pavan, M. 1956. Chim. Ind. (Milan). 38, 1015. von Eibl-Eibesfeldt, I. 1949. Ezperientia 5, 236. von Eibl-Eibesfeldt, I. 1962. 2. l’ierpsychol. 19, 385. von Frisch, K. 1941. 2. Vergleich. Physiol. 29, 46. Wilson, E. 0.1958.Psyche 65, 41. Wilson, E. 0.1959. Science 129, 643. Wilson, E. 0.1965. Science 149, 1064. Wilson, E. O.,and Bossert, W. H. 1963. Recent Progr. Hormone Res. 19, 673. Zander, E. 1951. “Handbuch der Bienenkunde 111. Der Bau der Biene.” Ulmer, Stuttgart.