The relationships between the brain, corpora allata, and oöcyte growth in the Central American locust, Schistocerca sp.—I. The cerebral neurosecretory system, the corpora allata, and oöcyte growth

J. Ins. Physiol., 1965, Vol. 11,pp. 13.5to 146. Pergamm Press Ltd.

Printed in Great Britain

THE RELATIONSHIPS BETWEEN THE BRAIN, CORPORA ALLATA, AND OOCYTE GROWTH IN THE CENTRAL AMERICAN LOCUST, SCHIsTO%ERCA SP.*-I. THE CEREBRAL NEUROSECRETORY SYSTEM, THE CORPORA ALLATA, AND OijCYTE GROWTH L. STRONG Department of Zoology, The University, Sheffield 10 (Receiered 12 August 1964) Abstract-In the adult female Central American locust, only slight changes occur in the amounts of stainable material in the neurosecretory system as the insect matures. However, the corpora allata undergo striking changes in volume and appearance during the course of maturation. Material from the median neurosecretory cells cannot be seen in the nerves which innervate the corpora allata, but lateral cell neurosecretion, which differs in histochemical properties from that of the median cells, can be detected in the nerves. Extirpation of the cerebral neurosecretory cells prevents oogenesis, but this operation also inhibits the activation of the corpora allata, which are essential for oacyte growth. While oacyte development is completely prevented by allatectomy, implanted corpora allata will not restore maturation. If immature corpora allata are implanted they do not become active, and if mature glands are used they rapidly lose their activity. In order to become and remain active, corpora allata must retain intact connexions with the central nervous system, INTRODUCTION

female desert locust Schistocerca gregaria, as in many other insects, oocyte growth is controlled by the pars intercerebralis-corpus cardiacum neurosecretory system and the corpora allata (HIGHNAM, 1963). It has been suggested that the two glands may have independent effects in S. gregaria: the neurosecretory system controls the concentration of protein in the haemolymph, and the corpora allata regulate its uptake by the developing oocytes (HILL, 1962; HIGHNAMet al., 1963). However, the way in which these cerebral endocrine glands are related is unknown; in particular, the mechanism of activation of the corpora allata is imperfectly understood. This investigation examines the relationships between the endocrine glands and oijcyte growth in another species of locust, special attention being paid to the control of the corpora allata. IN THE

* At present there is some uncertainty concerning the exact name of this species. It is reported to be Schistocercu paranerz& Burro. (Schistocercu cancellatu Forsk.), but until the problem is clarified the locust will be referred to by its general name, the Central American locust. 135

136

L. STRONG MATERIAL

AND

METHODS

Adult female locusts, supplied as last-instar hoppers by the Anti-Locust Research Centre, are reared in a constant-temperature room at 36”C, under alternating periods of 12 hr light and 12 hr darkness. Humidity is not controlled but varies little. The crowded locusts are reared in 50 1. cages and fed fresh lettuce and dry bran daily. Histological techniques Heads are fixed under reduced pressure for 24 hr in aqueous Bouin’s fluid. After dissecting away the cuticle and mandibular muscles, the complex of brain, gut, and endocrine glands is dehydrated, cleared, and embedded in paraffin wax (m.p. 56°C). Serial sections 10 TVthick are stained with aldehyde fuchsin (EWEN, 1962a). Aldehyde fuchsin has been adopted as the routine stain since chromium haematoxylin phloxine frequently yields poor results. Slides are stained in batches of twenty at a time to ensure the uniform treatment of both experimental and control sections. The sizes of corpora allata are measured by drawing every third section of a gland under a camera lucida and measuring the areas of the drawings with a disk planimeter. The volume is then calculated from the formula: Volume = 3 x total area of drawn sections x section thickness. This method is often criticized because of differential tissue shrinkage, but considering the differences found between the volumes of the corpora allata of the control and operated locusts in these experiments, its use is justifiable. Nuclear diameters are read directly from a calibrated eyepiece micrometer. Surgical techniques Cerebral neurosecretory cells are cauterized in locusts which have been anaesthetized with carbon dioxide and mounted in a clamp. A rectangular window is cut in the frons, and the pars intercerebralis of the brain exposed by removing the cephalic air sacs. The required areas are then destroyed with an electrically heated cautery needle. The flap of cuticle is sealed over the wound with paraffin wax. Sham operated locusts are treated similarly, but the needle is not heated. Allatectomy is performed with apparatus and method described elsewhere (STRONG, 1963). Sham operated locusts are treated like the operated locusts, but the glands are merely pulled with the forceps. To implant corpora allata, the glands are removed from donor insects and inserted, by means of forceps, into the abdomen or head capsule of the recipient. Sham operated locusts receive pieces of fat body. The allatectomy apparatus and instruments are sterilized before each operation in ‘Novasapa’ (Pharmaceutical Manufacturing Co.). In this way mortality can be kept low. Operated insects are always observed after treatment to ensure that they feed normally, and at the end of an experiment the success of cautery or allatectomy is always checked by histological methods. Maturation The degree of maturity of experimental females is estimated from the condition of the ovaries which are examined fresh under saline containing 7.5 g NaCl, 0.375 g KC1 (STAAL, 1961). The presence of oviducal secretion is noted, and the lengths of the terminal oocytes measured with callipers; the numbers of oiicytes and resorption bodies are counted. RESULTS In the Central American locust the structure of the ovaries and the cycles of oijgenesis are very similar to those in Locusta migratoria migratorioides (SINGH, 1958) and S. gregaria (SINGH, 1958; LUSIS, 1963), and therefore require only brief

BRAIN,

CORPORA

ALLATA,

AND

OijCYTE

GROWTH

IN THE

CENTRAL AMERICAN LOCUST-I

137

mention.

Each ovary comprises about fifty ovarioles arranged along longitudinal oviducts on either side of the midline. The terminal oocytes develop in synchronous batches. In the immature female the oviducts are thin and empty; the terminal oocytes are less than 1 mm in length and lack yolk. If males are present the females begin to mature in about 2 weeks (STRONG, 1964), pink secretion appearing in the Vitellogenesis begins when the oiicytes are some 2 mm long and oviducts. continues until they measure 7 mm, when they are ovulated into the oviducts. Although there are more than 100 ovarioles, 100 eggs are never laid; many oocytes are resorbed by the follicle cells. The penultimate oiicytes do not exceed 2 mm until the terminal oocytes are laid. After oviposition the new terminal oocytes begin to grow, the whole process being continuous. The structure of the endocrine system

The relationships of the endocrine system to the central and stomatogastric nervous systems are shown in Fig. 1. Two collections of neurosecretory cells, a large median group and a smaller lateral group, are found within each half of the The combined axons of each median group constitute a nerve, protocerebrum. the nervus corporis cardiaci 1, which traverses the brain and crosses its partner in the midline. The nerve then leaves the brain and loops ventrally to enter its respective corpus cardiacurn. The nervus corporis cardiaci 2, which originates in each lateral neurosecretory cell group, passes directly through the brain and descends ventrally to its corpus cardiacum. The nerves are not as straight as they appear in Fig. 1; the dissection was stretched to expose the parts. The paired corpora cardiaca, lying posterior to the brain, are in close contact with the hypocerebral ganglion and are closely applied to the walls of the aorta. A fine nerve, the nervus corporis allati 1, leaves the posterior face of each corpus cardiacurn and runs to the adjacent corpus allatum. Each corpus allatum is innervated by another nerve, the nervus corporis allati 2, from the suboesophageal ganglion. The paired corpora allata are egg-shaped glands which lie against the ventrolateral wall of the gut, just posterior to the circumoesophageal commissures. The fine structure of the endocrine system corresponds almost exactly with that of S. gregariu (HIGHNAM, 1961); only occasional additional features have been observed and will be mentioned, since they are relevant to the experimental work. The nervus corporis allati 1 is derived mainly, if not completely, from the

nervus corporis cardiaci 2. There is a considerable number of non-neurosecretory axons in the nervus corporis cardiaci 2; many of these pass into the nervus corporis allati 1. Similarly, there are many non-neurosecretory axons in each nervus corporis cardiaci 1; the whole ventral region of the nerve is devoid of neurosecretion. On entering the corpus cardiacurn, the ventral non-neurosecretory portion branches from the remainder of the nerve and descends into the hypocerebral ganglion. There is no evidence that the nervus corporis cardiaci 1

L. STRONG

138

contributes to the nervus corporis allati 1. Median cell neurosecretion is never seen in the nervus corporis allati 1. Median cell neurosecretion differs histochemically from that of the lateral cells. Median neurosecretion stains with aldehyde fuchsin if the sections are oxidized for only 1 min, while lateral neurosecretion stains only if the sections are oxidized for longer than 1 min. Furthermore, the lateral cells hardly stain with chromium haematoxylin phloxine, but the median cells stain blue-black or red. It is difficult to detect lateral cell secretion within the axons of the nervus corporis cardiaci 2 since the material does not form large granules typical of the median cell secretion. The neurosecretory system during maturation

Females were killed at various stages of development and the neurosecretory systems examined. In females with undeveloped ovaries. Females remam immature for about the first 2 weeks of adult life. The majority of the A-cells in the median group contain much stainable material, but occasional cells containing scattered granules are found. The nuclear diameters of the cells containing scattered granules are significantly greater than those of the full cells (Table 1). TABLE ~-VARIATION IN THE NUCLEARDIAMETEROF A-CELLS DURINGTHE SECRETORY CYCLE Number of observations Cells containing scattered granules

150

Cells full of secretion

150

Nuclear diameter 11.7+0.9

Comparison of means /.h

P-c 0.001 9.7 + 0.9 /.L

The abundant material in the corpora cardiaca is distributed evenly throughout the gland. The lateral neurosecretory cells undergo no visible changes during this period, and no material appears in the nervi corporis cardiaci 2 or in the nervi corporis allati 1. In females with developing ovaries. Ovarian maturation begins in about 2 weeks, but this is not correlated with any change in the amount of material within the neurosecretory system. The appearance of the median A-cells is unchanged, but the secretion in the corpora cardiaca lies mainly around the wall of the aorta. There are no changes in the amounts of stainable material as the oocytes grow to 5 mm in length. As maturation proceeds females accept courting males, but no differences are found between the neurosecretory systems of females killed before, during, and after copulation. In females with fully developed ovaries. As oiicyte growth is completed the amount of A-material in the median cells, nervi corporis cardiaci 1, and corpora cardiaca increases. Frequently there is a large accumulation of secretion in the

br olin-...

fr

gonglion FIG. 1. Dissection of the head capsule showing the anatomical relationships between the central nervous system, the stomatogastric nervous system, and the endocrine glands.

|

,,

T h e photomicrographs are secuons of material which has been fixed in aqueous Bouin's solution and stained with paraldehyde fuchsin. FIG. 2. Section through the brain of a mature female locust, showing a large accumulation of neurosecretory material in a nervus corporis cardiaci 1, where the nerve appears to be constricted by the perineurium. FIG. 3. Section through a nervus corporis cardiaci 2 of a female with eggs in the oviducts, showing accumulated droplets of lateral cell neurosecretion. FIG. 4. Section through a nervus corporis allati 1 of a female with eggs in the oviducts, showing accumulated droplets of lateral cell neurosecretion. FIG. 5. Cautery of the pars intercerebralis: section through a corpus allatum of an operated locust, showing tightly packed nuclei and sparse cytoplasm. FIG. 6. Cautery of the pars intercerebralis: section through a corpus allatum of a sham operated locust, showing scattered nuclei and abundant cytoplasm.

BRAIN, CORPORA ALLATA, AND ObCYTE GROWTH IN THE CENTRAL AMERICAN LOCUST-I

139

nervus corporis cardiaci 1 just within the perineurium, which appears to constrict the nerve (Fig. 2). While the eggs are in the oviducts, secretion often accumulates in the nervi corporis cardiaci 2 (Fig. 3) and nervi corporis allati 1 (Fig. 4). This material has the staining properties of lateral cell neurosecretion; it becomes very noticeable if the eggs are retained in the oviducts for some time. During oviposition there is no great reduction in the amount of stainable material in the neurosecretory system. Instead, as the next gonadotropic cycle begins, A-cells containing scattered granules slowly appear. Net changes are not observed in the amounts of B-material: neither A- nor B-material appears in the nervi corporis allati 1 at any stage. The corpora allata during maturation These glands undergo marked changes in structure matures (Table 2). TABLE

2----VARIATION

and volume as the female

IN CORPUS ALLATUM VOLUME WITH ObCYTE GROWTH

Stage of development

Number of observations

Newly emerged females, oacytes undeveloped

14

3-4 week females, oiicytes 5 mm long

10

Mature females, eggs in oviducts

Volume of corpus allatum (10s rS)

Comparison of means

P
20.52 f 0.45

I

P
The corpora allata of the newly emerged female are small with tightly packed nuclei and sparse cytoplasm. Towards the end of the second week the amount of cytoplasm increases, and the nuclei become dispersed. No mitoses are seen in the glands. Oviducal secretion becomes noticeable when the corpora allata measure 15 x lo6 p3, further growth of the glands being correlated with rapid growth of the terminal oijcytes. The corpora allata reach their greatest size when the oiicytes are 5 mm in length, but diminish in volume as the oijcytes grow from 5 to 7 mm. The glands remain small as long as the eggs are retained in the oviducts, and it is at this stage that lateral cell neurosecretion can be seen in the nerves to the corpora allata. After oviposition the glands begin another growth cycle, and the second batch of oiicytes develops. The cycles of the two glands are not perfectly synchronized, one gland often becoming active, or inactive, before the other. Surgical experiments Only slight changes occur in the neurosecretory system of the Central American locust as the oiicytes grow. However, lateral cell neurosecretion has been seen in

L. STRONG

140

the nerves supplying the corpora allata and may therefore play some part in the control of these glands. It became necessary to test the importance of the cerebral neurosecretory cells for oiicyte growth and corpus allatum activation. Cautery of the cerebral neurosecretory cells. The dorsal part of the pars intercerebralis of the brain was cauterized in twenty-four 3-day-old females, and a further eight were sham operated. Only ten operated females survived for 21 days, when they were killed and examined together with the sham operated locusts (Table 3). Examination of serial sections confirmed that the median cells of the operated females were extirpated but that, in addition, the lateral regions were destroyed. TABLE

~-THE

EFFECT OF CAUTERY GOCYTE

OF THE

GROWTH

PARS INTERCEREBRALIS

AND

CORPUS

ALLATUM

Operated locusts Number of locusts Length of terminal oijcytes (mm) Volume of corpus allatum (lo8 ps)

10 1.12 f 0.21 9.17 * 1.14

All females killed 21 days after emergence;

operations

OF IMMATURE

FEMALES

ON

VOLUME

Sham operated locusts

5.1 23.96

8 + 0.45 f 0.98

performed

Comparison of means

P-c 0.001 P-c 0.001

3 days after emergence.

In the operated females oijcyte growth is prevented, while the control females mature normally. The nervi corporis cardiaci and corpora cardiaca of the operated insects are devoid of secretion, or contain isolated droplets, and the corpora allata are small and inactive (Fig. 5). The corpora allata of the sham operated locusts are large with scattered nuclei (Fig. 6). There are other differences between the two groups: the cuticles of the operated locusts are thin and soft, their fat bodies are completely exhausted, and their haemolymph volumes are so large that haemolymph spurts from the necks when the locusts are decapitated. Cautery of the brain evidently violently disturbs metabolism. Since this operation prevents the corpora allata from becoming active, the lack of oiicyte growth in the cauterized females could be due to inactive corpora allata. It was therefore necessary to test the importance of these glands for oijcyte growth. Allutectomy. Thirty 3-day-old females were allatectomized, and a further five females sham operated. Twenty-eight operated and four sham operated locusts lived for 21 days, when they were killed and examined (Table 4). Allatectomy was incomplete in one insect (not used in the calculation of the mean). No other structures had been disturbed in the allatectomized females. The oocytes of the allatectomized locusts are less than 2 mm long, the length at which yolk is normally deposited, while the sham operated locusts develop the operated females will not mix with courting males. rapidly. Furthermore, There are no visible differences between the amounts of material in the neurosecretory systems of the two groups, and no stainable secretion accumulates in the axons of the cut nervi corporis allati 1.

BRAIN,CORPORAALLATA,AND

Ten 3-week-old females were allatectomized, and six were After 7 days the surviving seven operated and six control locusts examined. Allatectomy was complete in all the operated insects. TABLE

~-THE

141

OijCYTBGROWTHINTHECENTRALAMF.RICANLOCUST-I

EFFECT OF ALLATECTOMY

Number of locusts Length of terminal oijcytes (mm)

OF IMMATURE

sham operated. were killed and

FEMALES ON ~OCYTE

GROWTH

Operated locusts

Sham operated locusts

Comparison of means

27 1.07 + 0.001

4 8.48* + 1.48

P < 0.001

All females killed 21 days after emergence; operations performed 3 days after emergence. * Some of these females had oviposited. Therefore the terminal oocyte length is expressed as the sum of first and second generation oiicytes.

The operation does not hinder maturation in the sham operated locusts, the mean terminal oijcyte length being 5.4 f 0.34 mm (second generation obcytes) and the percentage resorption 37.4 f 2.7, a normal figure for second generation oiicytes. However, the operated females are very different. A few insects with almost fully developed oocytes at the time of operation oviposit 2-3 days after allatectomy, but the new terminal oocytes do not grow; the small amount of yolk deposited in these oocytes during the maturation of the first generation is resorbed. In the other insects the developing oocytes are completely resorbed. No differences are observed between the amounts of material in the neurosecretory systems of the two groups, and no secretion is found in the cut nervi corporis allati 1. Further experiments were designed to investigate whether isolated corpora allata would restore oijcyte development in allatectomized females. Implantation of corpora allata into allatectomized females. Twelve 3-day-old females were allatectomized and the corpora allata replaced in the abdomen, while five females were sham operated (series 1). Fourteen 3-day-old females were allatectomized and the glands replaced under the longitudinal tracheae of the head capsule, while five females were sham operated (series 2). Eleven operated females from series 1 and ten operated females from series 2 survived for 21 days, when they were killed and examined (Table 5). One insect in series 1 was incompletely allatectomized and was discarded from the series. In the operated locusts ovarian development is inhibited. Oviducal secretion can be detected in occasional ovaries, and some oiicytes contain traces of yolk. However, these oijcytes do not grow beyond 2 mm in length, and all show signs of resorption. Maturation in the sham operated locusts is unaffected by the treatment. The implanted corpora allata remain small and wrinkled, although they become tracheated. Even when the implants are found tucked under the aorta and in contact with the corpora cardiaca, they remain small.

142

L.

STRONG

Isolated corpora allata do not become active in the usual way and will not restore o6cyte growth in allatectomixed females. However, mature corpora allata may

remain active upon isolation.

This was investigated further.

TABLE S-THE EFFECTOF IMPLANTATION OFCORPORA ALLATAINTOIMMATURE ALLATECTOMIZED FEMALESON ij0CYTE GROWTH

Number of locusts in series 1 Length of terminal o&ytes (mm) in series 1 Number of locusts in series 2 Length of terminal oacytes (mm) in series 2

Operated locusts

Sham operated locusts

10

5

1.34kO.12 10 1.5

kO.06

5.6

+0+4

Comparison of means

P<0ao1

5 7.74 rt 0.68

P
All females killed 21 days after emergence; operations performed 3 days after emergence.

Each of eight 14-day-old females, which had been allatectomized at 3 days, received abdominal implants of two corpora allata from females with well-developed ovaries. After 7 days the surviving seven locusts were killed and examined. Pink secretion is produced in all the ovaries, but vitellogenesis begins in only a few, three females having yolkless oiicytes less than 2 mm long. Yolk-filled oijcytes occur in the remaining ovaries, but are resorbed before they exceed 3 mm in length. The implanted corpora allata are richly tracheated, but become small and wrinkled. Active corpora allata therefore lose their activity when isolated from the central nervous system and will not restore oiicyte development in allatectomized females. DISCUSSION

Histological signs of secretory activity in neurosecretory cells can be correlated with oScyte development and oviposition in many insects (see JOHANSSON(1958) for review), exceptions being Leucophaea maderae (ENGELMANN, 1957) and Oncopeltus fasciatus (JOHANSSON, 1958). It is not possible to correlate neurosecretory events with maturation in the red locust Nomadacris septemfasciata; in fact there are no obvious differences between the neurosecretory systems of diapausing and maturing adults (STRONG, unpublished observations). In the Central American locust only slight changes occur in the neurosecretory cells and corpora cardiaca while maturation is proceeding, but the changes are in no way as clear as those observed in S. gregaria (HIGHNAM, 1962). This observation does not invalidate the possibility that neurosecretion is being produced and released while the oacytes are growing, since the amount of material in a stained cell is not necessarily a direct measure of the cell’s activity. In a neurosecretory cell there are at least three events; the cell synthesizes material which is transported along the axon and released from the end of the axon. Only

BRAIN, CORPORA ALLATA, ANDObCYlRGROWTH IN THECENTRAL AMERICAN LOCUST-I the

143

resultant of these processes is observed in a histological preparation and, therefore, the appearance of the stained cell will depend entirely upon the extent to which the processes overlap. In some insects the cycles appear to follow one another in a regular manner, so that a cell may synthesize and accumulate material which is later transported down the axon often leaving a vacuolated cell (ARW and GABE, 1953a, b, 1954; ARVYet al., 1953; DUPONT-RAARE,1952; FORMIGONI,1956; HERLANT-MEEWISand PAQUET, 1956; EXVEN,1962b), or the processes may be In developing females of S. gregaria the median cells produce, concurrent. transport, and release material rapidly; these cells contain little stainable material in the form of scattered granules (HIGHNAM, 1962). Conversely, cells may be synthesizing and releasing material while they appear to be full of secretion (WIGGLESWORTH,1940). This occurs also in the lateral neurosecretory cells in the Central American locust (STRONG, 1964). In Calliphora erythrocephala, synthetically active neurosecretory cells have larger nuclei than those of inactive cells (LEA and THOMSEN,1962). Perhaps in the Central American locust the neurosecretory cells with scattered granules are synthetically more active than the full cells. However, until it is possible to distinguish between the cellular rates of synthesis, transport, and release of neurosecretory material it is unwise to attach great physiological significance to the histological appearance of neurosecretory systems. Furthermore, there is no certainty that the aldehyde fuchsin-positive material is the hormone itself or that if the stainable material is a carrier substance it is produced only when the hormone is produced. There may be other neurosecretory cells in the brain which do not become apparent with routine stains. Experimental results are also controversial. Oijgenesis in Rhodnius pro&us (WIGGLESWORTH,1936) and Platysamia cecropia (WILLIAMS, 1952) proceeds in the absence of the brain, but in C. erythrocephala the median neurosecretory cells are essential for maturation (THOMSEN, 1952). In phasmids (DUPONT-BAARE,1952, 1954) and O.ficiatus (JOHANSSON,1958), extirpation of the median neurosecretory cells merely reduces the total fecundity of the female. Ligation of the neck, anterior to the corpora allata, prevents oijgenesis in AZdes aegypti (GILLETT, 1956), and destruction of the pars intercerebralis in S. gregariu (HIGHNAM, 1962) and L. m. m&ratorioides (GIRARDIE,1963) inhibits maturation. Cautery of the pars intercerebralis of the Central American locust prevents oijcyte growth and causes other metabolic disturbances, the effect upon water content being of particular interest. But the corpora allata remain inactive after cautery, and since they are of great importance to oijcyte growth, the effect of cautery upon maturation may be due to the inactive corpora allata alone. In S. gregaria (HIGHNAM, 1962), and L. m. migratorioides (GIRARDIE, 1963) also, cautery of the brain prevents corpus allatum activation. The lateral neurosecretory complexes control the activation of the corpora allata in the Central American locust (STRONG, 1965). Since these lateral areas are destroyed by cautery of the pars intercerebralis the inactive corpora allata of cauterized locusts cannot be attributed to the destruction of the median neurosecretory cells.

144

L. STRONG

In many female insects the corpora allata are necessary for vitellogenesis (WIGGLESWORTH, 1936; PFEIFFER, 1939; THOMSEN, 1942; SCHARRER, 1946; JOHANSSON, 1958), but in the Central American locust the glands are essential for oijcyte growth. The oijcytes of allatectomized females do not grow to 2 mm, at which point vitellogenesis normally begins. The lack of development in these females resembles the condition found in the ovaries of diapausing adults of N. fusciuta, in which the corpora allata are very small (STRONG, unpublished observations). In S. gregariu also allatectomy inhibits oijcyte growth (HIGHNAM et al., 1963). The removal of corpora allata from maturing females of the Central American locust causes oijcyte resorption, as in many other insects (WIGGLESWORTH, 1936; THOMSEN, 1942; JOHANSSON, 1958; HIGHNAM et al., 1963). However, some of the females oviposit 2-3 days after the operation; their oScytes must have been some Either the oijcytes can complete the final stages 6 mm long at the time of operation. of development in the absence of the corpus allatum hormone, or the hormone titre in the haemolymph remains sufficiently high to complete oacyte development. The latter explanation is more likely since the corpora allata normally diminish in activity before the completion of oijcyte growth. Isolated inactive corpora allata do not become active or restore oijcyte development in allatectomized females of the Central American locust. Occasional ovaries produce a little oviducal secretion and traces of yolk, suggesting that the corpora allata become slightly secretory-probably activated by the operation. But the isolated glands soon lose their activity, and the yolk is resorbed. If active corpora allata are isolated they soon become inactive and non-secretory, although fresh tracheal connexions are established. In S. gregaria six active corpora allata are incapable of restoring normal oijcyte development in allatectomized females (HIGHNAM et al., 1963). Isolated corpora allata are capable of secretion at a minimum level in L. m. migrutorioides (STAAL, 1961) and differ in this from the glands of the Central American locust. Possibly, when the nerves to the corpora allata are cut in L. m. migratorioides the corpora allata are stimulated to secrete temporarily. Since STAAL (1961) worked on hoppers with an instar of only 5-6 days, a small amount of hormone may have been effective and given the impression of a low level of secretion. Since isolated corpora allata do not become active or secretory in the Central American locust, further work has been directed towards elucidating the function of the connexions between the glands and the brain. These results will appear in the second paper of this series. Acknowledgements-1 am grateful to Professor I. CHESTERJONES,in whose Department this work was carried out, for the provision of many facilities; to Dr. K. C. HIGHNAM for his supervision and continual encouragement; and to the Anti-Locust Research Centre, This work was carried out during the tenure of a London, for the provision of locusts. D.S.I.R. research studentship.

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