Anatomy and ultrastructure of the piercing-sucking mouthparts and paraglossal sensilla of Frankliniella occidentalis (Pergande) (Thysanoptera : Thripidae)

Int. J. Insect Morphol. & Embryol., Vol.21, No. 1, pp. 17-35, 1992

0020-7322/92 $5.00+ .00 © 1992PergamonPressplc

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ANATOMY AND ULTRASTRUCTURE OF THE PIERCING-SUCKING MOUTHPARTS A N D PARAGLOSSAL SENSILLA OF FRANKLINIELLA OCCIDENTALIS (PERGANDE) (THYSANOPTERA • THRIPIDAE)

WAYNE B . HUNTER a n d DIANE E . ULLMAN University of Hawaii at Manoa, Department of Entomology, 3050 Maile Way, Gilmore Hall, Rm 310, Honolulu, HI 96822, U.S.A.

( A c c e p t e d 11 N o v e m b e r 1991)

Abstract--Scanning and transmission electron microscopy are used to elucidate the internal morphology of the feeding structures of the western flower thrips, Frankliniella occidentalis (Thysanoptera : Thripidae). The position of the single mandible and paired maxillary stylets, relative to one another within the mouthcone, are shown through sequential thin sectioning. Observation of thin sections of all 3 stylets demonstrate them to be innervated. Within the mandibular stylet are 3 dendrites extending its length, and 2 lateral dendrites in its wall. Each maxillary stylet contains 4 dendritic canals, each with one dendrite. The location of the cibarium within the mouthcone is examined and the attachments of its dilator muscles are shown. One to 3 dendrites, varying in functional type, are found in each paraglossal sensillum. The ultrastructural evidence presented suggests that some paraglossal sensilla have a dual chemosensory and mechanosensory function, while others are apparently solely mechanosensory. The significance of these findings relative to the feeding mechanisms of thrips and the possible function of stylet and paraglossal sensory structures in host location, feeding site selection and host choice are discussed.

Index descriptors (in addition to those in title): Feeding, chemosensory, mechanosensory, behavior, styler innervation, cibarium, mouthcone.

INTRODUCTION THRIPS c a u s e d i r e c t e c o n o m i c d a m a g e t o a v a r i e t y o f f o o d , f i b e r a n d o r n a m e n t a l c r o p s worldwide

(Lewis,

1973;

Heming,

1985;

Strassen,

1986).

Crop

losses,

due

to

t r a n s m i s s i o n of t o m a t o s p o t t e d wilt virus ( T S W V ) by thrips are f r e q u e n t l y s e r i o u s a n d

Abbreviations used in figures: C = compound eye; Cc = cuticular collar; cd = central dendrite; Cib = cibarium; CI = frontoclypeus; CIm = clypeolabral membrane; Cm = cibarial muscles; COe = circumoesophageal passage; Cp = cuticle of peg; d = dendrite; dc = dendritic canal; ephs = epipharyngeal sclerite; Fc = food channel; GI = glossae; Hyp = hypopharynx; Lb = labium; Lbr = labrum; ld = lateral dendrite; lp = labial palp; Mc = mouthcone; md = mandibular stylet; Mds = mandibular sheath; mp = maxillary palp; mx = maxillary stylet; Oe = oesophagus; Pg = paraglossa; PI = prothoracic leg; s = salivary duct; Sg = suboesophageal ganglion; Sp = simple pore; Spg = supraoesophageal ganglion; St = maxillary stipes. 17

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W.B. HUNTERand D. E. ULLMAN

have stimulated much interest in the role of thrips as virus vectors (Sakimura, 1962; Paliwal, 1976; Amin et al., 1981; Cho et al., 1984, 1987; Greenough et al., 1985; Yudin et al., 1986; Allen and Broadbent, 1986). In Hawaii, Franklienella occidentalis, the western flower thrips (WFT), is thought to be the primary TSWV vector species. Hence, this thrips species is the focus of our investigation (Cho et al., 1987). Management of thrips vector species and the spread of TSWV have become important loci of research programs around the world (Peters et al., 1991). Knowledge of the feeding behavior of thrips, especially for major vector species like the WFF, should provide an important foundation for developing management strategies. For example, plant breeding programs aimed at developing thrips- and TSWV-resistant plant varieties may benefit by directing their efforts at incorporation of plant characteristics with the potential for altering the chain of behaviors leading to thrips feeding. An understanding of the morphology, innervation and function of the feeding structures of thrips could provide a basis for elucidating and manipulating feeding behavior. Recently, the internal and external morphology of the mouthcone and feeding structures of the W F F and of several other thrips have been examined (Heming, 1978; 1985; Chisholm and Lewis, 1984; Honomichl, 1982; Wiesenborn and Morse, 1985, 1988; H u n t e r and Ullman, 1989; Moritz, 1989). On the paraglossae of the WFT, 3 morphologically distinct sensilla have been described: sensilla basiconica with cuticular collar, sensilla basiconica without a collar, and sensilla trichoidea (Chisholm and Lewis, 1984; Hunter and Ullman, 1989). Chisholm and Lewis (1984) proposed, based on external morphology, that these structures may have a chemosensory role. Later, H u n t e r and Ullman (1989) provided evidence that these sensilla were innervated, although no further evidence regarding fine structure or other functions were presented. Previous works have supplied important descriptions of the musculature, structure and innervation of the mouthcone and stylets using light microscopy, and reconstructed 3-D illustrations (Peterson, 1915; Reyne, 1927; Wardle and Simpson, 1927; Pesson, 1951; Risler, 1957; Mickoleit, 1963; Heming, 1978; Honomichl, 1982; Moritz, 1982). This study is the first to examine the ultrastructure of the paraglossal sensilla and stylet-borne sensilla with TEM, as well as presenting a view of the internal aspects of the anterior alimentary canal at the electron microscopy level. The relevance of these findings to the feeding behavior of thrips and other studies of ultrastructure are discussed.

MATERIALS AND METHODS All thrips examined were adults from colonies of Frankliniella occidentalis (Pergande) maintained on bean pods (Phaseolus vulgaris L.), in clear plastic containers, held at 25°C with continuous light. Species identificationwas confirmedby Mr D. Tsuda, Department of Entomology, Insect DiagnosticClinic, University of Hawaii at Manoa. Three methods were used to prepare thrips for examination by scanning electron microscopy(SEM). In the first method, thrips were dehydrated in an ethanol series ending with 2 x 30 rnin changes in 95% EtOH followed by 3 x 30 rain changes in 100% EtOH. Specimenswere then critical-pointdried (Autosamdri 180, Tousimis Research Corp., Rockville, MD 20852, U.S.A.), mounted on SEM stubs with silver print paint (Sigmund-Cohn Hed Wire Inc., Mt. Vernon, NY 10553, U.S.A.), sputter coated, and examinedwith a Hitachi S-800 (Field Emission Scanning Electron Microscope,Hitachi Ltd, Japan). In the second method, thrips were dehydrated and critical-pointdried as above, mounted dorsal side down on SEM stubs and covered with a light layer of silver print conductivepaint. A teflon-coated stainless steel razor blade was then lightly drawn across the insect and the cut edges carefully peeled apart to expose the interior. The specimens were then sputter coated and examined. In the third method, some dehydrated, critical-point dried specimens were plasma etched ("Plasma Prep", Structure Probe Inc., Tegal Co., California, U.S.A.), sputter coated, and examined using previouslydescribed techniques (Hunter and Ullman, 1989).

Anatomy and Ultrastructure of Frankliniella occidentalis

19

A

1.6pm Fro. 1. Frontal view of head, showing compound eyes (C), frontoclypeus (C1), clypeolabral membrane (Clm), labium (Lb), labrum (Lbr), labial palps (lp), mandibular sheath (Mds), maxillary palps (mp), maxillary stipes (St), paraglossae (Pg), and prothoracic leg (P1). Fro. 2. Transmission electron micrograph (TEM) cross-section through maxillary palp (rap) showing 6 dendritic sheaths within (arrow) one for each palpal sensillum. FIG. 3. TEM cross-section through labial palp (lp) showing 4 dendritic sheaths within (arrow).

Thrips were also prepared for sequential sectioning and transmission electron microscopy (TEM) using the techniques of Ullman et al. (1989). Sensillum neurons of plasma-etched specimens (SEMs), were measured and compared with TEMs of similar neurons. Thirty adult thrips were prepared for viewing with TEM, 10 of them with sequential sections of the entire head. RESULTS A f r o n t a l view o f t h e m o u t h c o n e o f t h r i p s r e v e a l s s e n s o r y s t r u c t u r e s o n t h e m a x i l l a r y ( m p ) a n d l a b i a l (lp) p a l p s , as well as p a r a g l o s s a l sensilla (Figs 1-4). T h e 10 sensilla

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Ft6. 4. Left paraglossa (Pg) showing l0 sensory pegs. Three morphologically distinct types are shown. Sensillum basiconica lacking cuticular collar (1, 6), s. basiconica with cuticular collar (4, 5, 7-9) and s. trichodea (2, 3, 10). FI~. 5. Thin section through sensory dendrites of left paraglossa (Pg), showing number of sensory dendrites innervating each sensillum. Only sensillum 6 shows a single large dendrite (d), (arrow), all other sensilla have 2 small and one large dendrite, similar to those of sensillum 5 (arrows).

present on each paraglossa (Pg) are of 3 distinct types: sensilla basiconica with and without cuticular collars, and sensilla trichoidea (Fig. 4). Each sensillum is innervated (Fig. 5) and except for sensillum 6, the size and number of dendrites within each sensillum is similar. Sensillum 6 (Fig. 5, arrow), a sensillum hasiconica without cuticular collar, has only one large dendrite, while each of the remaining sensilla have one large dendrite, c. 0.27 Ixm, and 2 smaller dendrites, c. 0.12 v~m. Examination by TEM of sequential, thin sections of the sensilla basiconica with collar revealed a single pore (Sp) at the tip of these sensilla (Figs 8, 9). However, examination with high magnification SEM techniques, allowing observations up to 80,000 x , did not reveal cuticular pores in the sides or tip of any of these sensilla (Fig. 6). Plasma etching and TEM of a similar sensillum permitted measurements of dendrite lengths and widths and demonstrated that at least one dendrite (d) extends nearly to the peg tip (Figs 6, 7). The single mandible (md) lies within a mandibular sheath (Mds) along the left-hand side of the mouthcone (Figs 1, 10). Partial removal of the labrum (Lbr) exposed the single, left mandibular stylet (md) and showed it to be attached to the exoskeleton at its

Anatomy and Ultrastructure of Frankliniella occidentalis

FIG. 6. Scanning electron micrograph (SEM) of a sensillum basiconica with cuticular collar (Cc), showing cuticle of peg (Cp). Paraglossa (Pg). FIG. 7. SEM of a sensillum basiconica after plasma etching. Cuticular collar and cuticle of peg have been etched away to reveal dendrites (d) within. Paraglossa (Pg).

21

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8

9

1.0pr FIG. 8. TEM thin sectionof a sensillumbasiconicashowingcuticularcollar (Cc), cuticleof peg (Cp), dendrite (d) and location of simple pore (Sp) at tip. FIG. 9. Higher magnificationTEM of tip of sensillum showing simple pore (Sp). Dendrite (d) terminates approximately0.18 vtm proximal to internal pore opening (tip of arrow).

base (Fig. 10, white arrow). The mandible (md) is approximately 69 wm long from its tip to where it starts to bend and enlarge. At this proximal point it is approximately 2 I~m in diameter. Observations by SEM, with magnifications up to 80,000 x , did not reveal evidence of external sensilla at the tip of the mandible. Internally, T E M observations of sequential thin sections, demonstrated the mandibular stylet to contain a central dendritic canal containing 3 dendrites (cd) that extend throughout its length and attach distally to its tip (Figs 11-13). Laterally, there are 2 smaller canals (ld) containing one dendrite each; these canals end proximal to the tip of the stylet (Figs 11, 12). When the labrum and epipharyngeal plate are removed, the position in which the paired maxillary stylets (mx) lie can be seen, relative to the position of the mandible (md) (Fig. 14). The maxillary stylets (mx) lie posterior to, and on either side of, the mandible (md) (Fig. 14). Distally, the maxillary stylets (mx) converge and when retracted, their tips interlock, for approximately 12 I~m, within the mouthcone (Fig. 15).

Anatomy and Ultrastructure of Frankliniella occidentalis

FIG. 10. SEM of anterolateral aspect of thrips mouthcone. Labrum (Lbr) and left maxillary stipes (St) have been slightly lifted to show where mandible joins to exoskeleton (white arrow) beneath clypeolabral membrane (Clm) and position of single mandibular stylet (md) within mandibular sheath. Maxillary palp (mp) and labium (Lh). Arrows denote mandibular groove in hypopharynx disrupted during dissection.

FIG. 11. TEM of proximal thin section through mandibular stylet, showing 3 central dendrites (cd), which extend length of stylet and 2 lateral dendrites (ld) within its wall. F1G. 12. TEM of intermediate level thin section through mandibular stylet, showing only 3 central dendrites (cd). FIG. 13. TEM of distal thin section of mandibular stylet, showing where 3 central dendrites attach to its tip.

23

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W.B. HUNTERand D. E. ULLMAN

FIG. 14. Front aspect of thrips mouthcone with labrum and epipharynx removed. Position of maxillarystylets(absent) (mx, arrows) is posterior to, and on either side of, mandibularstylet (md). Fl~;. 15. Tip of thrips mouthcone with labrum (Lbr) lifted to expose where maxillarystylets (mx) converge and interlock for approximately 12 I~m within mouthcone, thus forming a feeding tube. Labial palp (lp).

When so interlocked, they form the feeding tube through which plant fluids are ingested. As they protract, the maxillary stylets (mx) fit together in a "tongue and groove" fashion which allows them to slide along each other in a longitudinal direction (Figs 16, 17). The wall of each maxillary stylet (mx) has one canal at its base which splits into 4 dendritic canals (dc) distally (Figs 18-20). Each canal houses one dendrite (d) (Fig. 21) which attaches distally to the tip of the stylet (Fig. 22, arrows indicate attachment sites). The mouthcone (Mc) was artificially compressed and the stylets exposed (Fig. 23). During thrips feeding, the mandibular stylet (md) pierces the plant epidermis and the maxillary stylets (mx) follow (Fig. 23). Transverse sequential sections examined by TEM, show the relative positions of all 3 stylets within the mouthcone (Figs 24-26, 28). At the tip of the mouthcone, where the maxillary stylets (mx) converge and interlock, the food (Fc) and salivary (s) canals also converge, the latter presumably emptying saliva directly into the single canal (Fc) between the maxillary stylets (mx) (Figs 24, 25). Distal, transverse sections show the stylets converging within the mouthcone, with the mandible (rod) anterior to the

Anatomy and Ultrastructure of Frankliniellaoccidentalis

25

FIG. 16. SEM of maxillary stylets (mx) cut to show food canal (Fc) created when these stylets are interlocked. FIG. 17. TEM of thin section through longitudinal tongue and groove interlocking mechanism of maxillary stylets (mx) and food canal (Fc) between them.

maxillary stylets (mx) and the food canal (Fc) of the hypopharynx (Hyp) (Fig. 24). Proximally within the mouthcone, the mandible (md) is lodged in a groove along the lefthand side of the cibarium (Cib) anterior to the maxillary stylets (mx), which are situated laterally and slightly posterior to the cibarium (Fig. 26). Removal of the frontoclypeal region reveals the position and size of the cibarium (Cib) within the mouthcone (Fig. 27). Dorsal cibarial dilator muscles (Cm) can be observed (Fig. 28). Food exits the cibarium (Cib), entering the pharynx and oesophagus (Oe), which passes between the supraoesophageal (Spg) and suboesophageal (Sg) ganglia, by way of the circumoesophageal (COe) passage and into the midgut (Figs 29, 30). The oesophagus (Oe) was nicked during dissection (Fig. 30).

DISCUSSION Innervation of paraglossal sensory structures and proposed functions Several recent morphological studies on various thrips have demonstrated the presence of paraglossal sensilla (Chisholm and Lewis, 1984; Wiesenborn and Morse, 1988; Hunter and Ullman, 1989; Moritz, 1989). External morphology and innervation of these structures have supported the hypothesis that these pegs serve a sensory function

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W . B . HUNTER and D. E. ULLMAN

FIG. 18. TEM of thin section through base of maxillary stylet (mx) showing single, large dendritic canal (dc) containing 4 dendrites (d) (white arrow). FIG. 19. TEM of thin section made slightly distal of section in Fig. 18, showing formation of individual dendritic canals (dc) within maxillary stylet (mx). Arrows point to 2 dendritic canals, one containing a single dendrite, other 3 dendrites. FIG. 20. TEM of thin section through maxillary stylet (mx), taken distal to section in Fig. 19, showing that each dendrite is becoming enclosed within a single dendritic canal (dc). FIG. 21. TEM of thin section through maxillary stylet (mx), distal to section in Fig. 20, showing each dendritic canal containing a single dendrite (d). FiG. 22, TEM of thin section through distal tip of interlocked maxillary stylets (mx). Arrows show where dendrites are attached.

Anatomy and Ultrastructure of Frankliniella occidentalis

27

Fic. 23. SEM of tip of mouthcone (Mc) which has been compressed to show stylets. During feeding, thrips pierce substrate with mandibular stylet (rod) which is rapidly followed by paired maxillary stylets (mx).

(Chisholm and Lewis, 1984; Hunter and Ullman, 1989). Chisholm and Lewis (1984) suggested that 3 morphologically distinct types of sensory pegs found on the paraglossae of Limothrips cerealium--sensilla basiconica with or without a cuticular collar and sensilla t r i c h o i d e a - - a p p e a r to be chemosensory, although they do not present direct evidence supporting this contention. The data we present, demonstrates the fine structure of paraglossal sensilla in the WF-I', and suggests that some of these sensory structures have a dual function (Figs 4, 5, sensilla 1-5, 7-10) serving as both mechanosensilla and chemosensilla. Sensilla with both mechanosensory and chemosensory functions are known to occur among other insect orders, such as the H o m o p t e r a (Foster et al., 1983; Backus, 1985; Zacharuk, 1985). Sensilla with dual mechanosensory/chemosensory function are usually innervated by more than one neuron, one neuron being larger than the others and ending at the base of the peg (a characteristic of mechanosensilla) (Mclver, 1975, 1985). Typically the other dendrites enter the peg and end near a pore at its tip, as is common among uniporous chemosensilla (Zacharuk, 1980, 1985). When dendrites of this nature are present in a

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FIG. 24. TEM of thin sections through mouthcone, showing that, at distal tip, where maxillary stylets (mx) converge, salivary canal (s) also converges and presumably empties directly into single canal, which also functions as food canal (Fc). Mandible (md) is anterior to maxillary stylets (mx), hypopharynx (Hyp) and glossae (GI) at this level. FIG. 25. TEM of intermediate level thin section through mouthcone, showing that as sections are observed more proximally, maxillary stylets (mx) are located more laterally and posteriorly to food canal (Fc), hypopharynx (Hyp) and mandible (md). FIG. 26. TEM of proximal level thin section through mouthcone, showing positions of single mandibular (md) and paired maxillary (rex) stylets relative to cibarium (Cib). Cibarial dilator muscles attach to epipharyngeal sclerite (ephs).

Anatomy and Ultrastructure of Frankliniella occidentalis

FIG. 27. Composite SEM of lateral aspect of head, clypeal region removed to reveal approximate position and size of cibarium (Cib) within mouthcone. Dorsal cibarial dilator muscles (Cm), compound eye (C), maxillary palp (mp) and labial paip (lp). F1G. 28. TEM of transverse thin section through cibarium (Cib), showing insertion of dorsal cibarial muscles (Cm) into epipharyngeal sclerite (ephs) and relative positions of single mandibular (md) and paired maxillary (mx) stylets.

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F16. 29. SEM of ventral aspect of prothorax and mouthcone (Mc). Mouthcone has been pushed upward to reveal oesophagus (Oe). Compound eye (C), maxillary palp (mp) and prothoracic leg (PI). FIG. 30. Higher magnification SEM of oesophagus (Oe) leaving mouthcone and passing between supraoesophageal (Spg) and suboesophageal ganglia (Sg) through circumoesophageal passage (COe). Oesophagus was nicked during dissection.

Anatomy and Ultrastructure of Frankliniella occidentalis

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sensillum, the structure is thought to have a dual function, responding to both leaf microtopography and plant surface chemicals (Foster et al., 1983). Furthermore, the appearance of these sensilla (Fig. 4, sensilla 4, 5, 7-9) is similar to previously described chemosensory organs (Schenk, 1903; Snodgrass, 1926; Peregrine, 1972). Careful examination of the fine structure of insect sensilla frequently provides information allowing predictions regarding their function (Slifer, 1970). This contention has been well supported by the research of numerous investigators (Mclver, 1975, 1985; Zacharuk, 1980, 1985), and provides a basis for our hypothesis that some paraglossal sensilla serve the type of dual sensory function described above. The most detailed ultrastructural evidence we provide is for the sensiUum basiconica with a cuticular collar. These sensilla basiconica are innervated by more then one neuron (Fig. 5), one characteristic of chemosensory function (Slifer, 1970). In the WF'I', of the 3 dendrites present, the 2 smaller ones appear to extend to the tip of the cuticular peg (Figs 7-9), further indicating a chemosensory function (Zacharuk, 1980, 1985). Perhaps the most compelling evidence that this sensillum has a chemosensory function, is the presence of a pore at the tip, to which the 2 smaller dendrites extend (Figs 8, 9). This pore could only be seen in thin sections with TEM. Visualization with SEM was probably prevented by exuded material obstructing the pore (Hansen and Heumann, 1971; Moulins, 1971; Dethier, 1972). The 3rd, larger dendrite, whose width is greater than that of the 2 smaller ones combined (Fig. 5), could not be visualized at the distal portion of the sensillum. Thus, it is thought to end near the base of the peg. This is characteristic of mechanosensitive dendrites present in some chemosensilla, such as certain uniporous peg sensiUum (Zacharuk, 1980, 1985). While less detailed ultrastructural information is available for the sensiUa basiconica without collar and sensilla trichoidea, cross-sections of these structures indicate that their fine structure is similar to that of the sensilla basiconica with collar described above (Fig. 5). Thus, it is likely that these structures also serve dual mechanosensory and chemosensory functions. Paraglossal peg number 6, a sensillum basiconica without collar, clearly differs from all others in its fine structure (Fig. 5). Innervation of this sensillum by a single, large dendrite strongly suggests a solely mechanosensory function (Mclver, 1975, 1985). By analogy to leafhoppers and planthoppers (Foster et al., 1983; Backus, 1985), the presence of mechanosensory and chemosensory organs on paraglossae of thrips, suggests that plant surface microtopography and chemistry probably have an important role in host choice and feeding site selection by thrips. Considering the likely importance of plant surface characteristics, future investigation of control strategies for thrips may want to focus more intensively on repellent, deterrent, and/or plant resistant properties at the leaf or flower surface which could influence thrips feeding.

lnnervation of thrips stylets The evidence we present, confirms the earlier findings of Heming (1978), that thrips stylets are innervated, and further illuminates the mechanism by which thrips control stylet direction and movement in response to sensory cues. Within both mandibular and maxillary dendritic canals (dc), the dendrites (cd, d) appear to be free-floating, except for an attachment distally to the tips of the stylets (Figs 11, 13, 19-22). These sensilla probably serve a mechanosensory function, though in Hemipterans and Homopterans, similar stylet sensilla have been reported to function as both mechanoreceptors and hygroreceptors (Wensler, 1974). These findings, in conjunction with known behavioral

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w.B. HUNTERand D. E. ULLMAN

observations and plant histological evidence, support the contention that thrips use both mandibular and maxillary stylets in conjunction to pierce particular cells, empty their contents and then to extend the latter deeper into plant tissues in response to mechanosensory cues (Wardle and Simpson, 1927; Sakimura, 1962; Lewis, 1973; Heming, 1978; Hunter and Ullman, 1989; Huckaba and Coble, 1991; Hunter et al., 1992; Ullman et al., 1992). The placement of thrips stylets in relation to one another, is unique among the Insecta. Unlike Hemipteran and Homopteran insects, thrips have only the left mandibular stylet (md) (Figs 10, 14). In addition, this stylet does not encircle the maxillary stylets, as do the 2 mandibular stylets present in Hemipteran and Homopteran insects. Instead, the mandibular stylet (md) of the WFT is positioned some distance from the maxillary stylets (mx), except at the tip of the mouthcone where all 3 stylets converge (Figs 24, 25). The mandibular stylet, which functions in piercing the plant epidermis, may also sense the firmness of a substrate during the piercing process, depending upon the degree of bending and stretching of the internal stylet sensilla. Additional cues regarding stylet movement may be provided by a proprioceptor, described by Honomichl (1982), at the base of the mandibular stylet of Thrips physapus L. The maxillary stylets (mx) of thrips are unique in that they have a single food canal (Fc) (Figs 16, 17), through which saliva must also pass. The opening of the salivary duct (s) is located where the maxillary stylets (mx) meet and interlock (Fig. 24). During feeding, plant cytoplasm is sucked up the food canal (Fc), passing into the cibarium (Cib) (Figs 24-28) and then entering the oesophagus (Oe) (Figs 29, 30). Excellent, overall descriptions of thrips feeding lend support to the above (Heming, 1978). Sensilla within the maxillary stylets probably function similarly to the stylet-borne sensilla of other piercing-sucking insects (Forbes, 1976, 1977; Backus, 1985), which are thought to monitor the movement of stylet tips through plant tissues by the bending of, and tension on, the dendrites (Wensler, 1974). Independent stylet movement among thrips species has been demonstrated (Heming, 1978; Chisholm and Lewis, 1984) and our findings provide, at least in part, the sensory basis for these movements. Furthermore, the presence of stylet sensilla in all 3 WFT stylets (Figs 11-13, 21) supports previous reports that independent stylet movement occurs and is a broad characteristic of thrips feeding. From these data, it appears that thrips feeding may be more analogous to that of leafhoppers, in which both maxillary stylets and mandibular stylets are innervated and independent stylet movement is possible (Backus, 1985). The probable importance of stylet sensilla in directing thrips stylet movements suggests plant anatomy may critically influence thrips selection of feeding sites on the leaf surface as well as within plant tissues.

Classification of thrips feeding behavior Our data and that presented by other investigators for thrips species in both the Tubuliferan and Terebrantian suborders, strongly support the classification of Thysanopteran feeding as piercing-sucking (Borden, 1915; Peterson, 1915; Reyne, 1927; Wardle and Simpson, 1927; Sakimura and Carter, 1934; Risler, 1957; Mickoleit, 1963; Mound, 1971; Heming, 1978, 1985; Moritz, 1982, 1989; Chisholm and Lewis, 1984; Evans, 1984; Wiesenborn and Morse, 1988; Borror et al., 1989; Hunter and Ullman, 1989; Ullman et al., 1989; Huckaba and Coble, 1991; Hunter et al., 1992; Ullman eta/., 1992) not rasping-sucking (Metcalf et al., 1962; Elzinga, 1987; Pedigo, 1989; Welter,

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33

1989). The presence of a cibarium (Cib) (Figs 26-28), which serves as a pump, and paired, interlocking maxillary stylets (mx) (Figs 16, 17), which form a food canal (Fc), are typical of other piercing-sucking feeding insects (Forbes, 1976; Backus, 1985). The observations we report, regarding the morphology and innervation of the feeding structures of thrips, adds significantly to this growing body of evidence. Furthermore, our findings provide a broader understanding of thrips feeding and the possible sensory mechanisms underlying plant host and feeding site selection. Innervation of the mandibular (md) and maxillary stylets (mx) (Figs 11-13, 21), strongly suggests that stylet movement in thrips is, in part, in response to reception of mechanosensory cues. Furthermore, placement and directional movement of these stylets are likely to be in response to plant cues. In addition, paraglossal sensory structures are innervated (Fig. 5) as previously suggested (Chisholm and Lewis, 1984; Hunter and Ullman, 1989). The anatomy of the paraglossal sensilla, multiple dendrites, and the presence of a simple pore (Figs 5-9), suggests mechanosensory and chemosensory functions. Thus, the mechanics of thrips feeding revolves around the likely selection of feeding sites in response to mechanical and chemical cues on the plant surface, as well as by ingestion of cytoplasm from plant cells beneath the epidermis (Mound, 1971; Heming, 1978, 1985; Chisholm and Lewis, 1984; Wiesenborn and Morse, 1985, 1988; H u n t e r and Ullman, 1989; Huckaba and Coble, 1991; H u n t e r et al., 1992; Ullman et al., 1991, 1992). New knowledge regarding thrips feeding mechanisms, influences current views on: (1) damage to leaves and fruits caused by direct feeding of thrips (Chisholm and Lewis, 1984; Huckaba and Coble, 1991), (2) assessment of related yield losses (Welter, 1989; Rosenheim et al., 1990; Welter et al., 1990) and (3) transmission of TSWV (Peters et al., 1991) and other plant viruses, like Ilarviruses (Sdoodee and Teakle, 1987, 1988). Furthermore, much may also be learned by comparing host finding, feeding site selection and feeding behavior by thrips, to better studied systems among piercing-sucking insects (Forbes, 1977; Markham et al., 1984; Backus, 1985). Acknowledgements--We are indebted to Marilyn Dunlap and David Fisher for facilitiesmade available, Tina

Carvalho and Delta Westcot for their assistance and guidance with electron microscopytechniques, and Karin Jacobsen for help with translations. Many thanks to Mark Berman for photographic assistance, and Lynn Lebeck and Nallur Kumar Krishna for reviewingthe manuscript. This investigationwas supported in part by grants from the Research Centers in Minority Institution award, RR-03061, from the Division of Research Resources, National Institutes of Health and by United States Department of Agriculture under CSRS Special Grant #88-34135-3593 managed by the PacificBasin Advisory Group (PBAG). This research was conducted in partial fulfillmentof Ph.D. requirements. This is Journal Series No. 3614 of the Hawaii Institute of Tropical Agriculture and Human Resources.

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