/. Embryo!, exp. Morph. Vol. 60, pp. 329-343, 1980 329Printed in Great Britain © Company of Biologists Limited 1980

The eye margin and compound-eyedevelopment in the cockroach: evidence

against recruitment

By MARK S. NOWEL1 AND PETER M. J. SHELTON1

From the Department of Zoology, University of Leicester

SUMMARYThe compound eye of the cockroach nymph grows from stadium to stadium by the

addition of new ommatidia to the growing edge of the eye. By a. series of transplant operationson Periplaneta americana and from SEM studies on Gromphadorhina portentosa it is shownthat the proliferating region of the eye margin is a budding zone. There is no recruitment oflarval head-capsule epidermis into the eye.

INTRODUCTION

During development of the insect compound eye, new ommatidia arecontinually added to one or more edges of the expanding structure. Thus theyoungest ommatidia are closest to the developing margin. The cells which areincorporated into differentiating ommatidia are derived from a proliferatingzone lying just within the eye margin and next to the epidermis of the headcapsule. The proliferating zone is regarded by Bodenstein (1953) as a persistentprimitive portion of the original eye anlage already present in newly hatchednymphs (Jorschke, 1914; Friza, 1928). The proliferating zone is thus an integralpart of the eye and is already determined to form eye. Bodenstein (1953) refersto the proliferating zone as the budding zone, and his conclusion was based onhistological evidence. An alternative hypothesis is that the 'eye field' extendsbeyond the eye margin into the epidermis which forms the head capsule. Theproliferating zone in that case would mark the position of an advancing frontat which cells are recruited into the eye margin where they subsequently divideand differentiate to form ommatidia. This recruitment hypothesis is based onthe results of transplant experiments where marked grafts of retina or head-capsule epidermis were exchanged between eye-colour mutants (White, 1961,1963; Hyde, 1972; Egelhaaf, Berndt & Kuthe, 1975; Lawrence & Shelton, 1975;Green & Lawrence, 1975; Nardi, 1977).

According to the first theory, 'there is no transformation of normal nymphal

1 Authors'' address; Department of Zoology, University of Leicester, Leicester LEI 7RH,U.K.

330 MARK S. NOWEL AND PETER M. J. SHELTON

ectoderm cells into sensory cells during postembryonic development' (Boden-stein, 1953). The second theory proposes that epidermal cells which at onestage secrete the cuticle of the head capsule are subsequently recruited andtransformed into eye cells. The object of this paper is to test both of thetheories, because to date none of the evidence is conclusive. On the one hand,Bodenstein's observations were histological and did not test for recruitment.On the other hand, the evidence for recruitment comes from transplantexperiments and is limited by the accuracy with which defined areas of tissuecan be excised and grafted.

In this paper, different types of grafting experiments on Periplaneta americanaare described which eliminate the ambiguities inherent in previous studies. Theresults argue in favour of the budding-zone hypothesis. There is no evidencefor recruitment.

In another approach, using the scanning electron microscope (SEM), theposition of the eye margin with respect to identified bristles near to the eyewas examined at successive moults of the cockroach Gromphadorhina portentosa.Some of these bristles are only one or two cells away from the eye margin, yetover a period of considerable eye growth during two intermoult periods theeye never approaches or envelopes the bristles. This provides a second line ofevidence in favour of the budding-zone hypothesis.

MATERIALS AND METHODS

Stocks of P. americana and G. portentosa were kept in standard laboratoryconditions. They were fed on rat pellets and water. Experimental animals werekept in small gauze-topped sandwich boxes. For surgery, recently moultedanimals at larval stages 3-5 were anaesthetized in small glass vials cooled onice for 10-20 min. They were then placed on a bed of plasticine and held downwith narrow strips of plasticine. Cuts in the integument were made using arazor-blade fragment (Gilette francais) supported in a pin vice. Sites wereprepared in host animals by removing cuticle and attached epidermis, and donorfragments were cut to fit. They were transferred using tungsten needles orwatchmakers' forceps. The grafts were held in place using insect wax (Krogh& Weis-Fogh, 1951) just above its melting point. Wild-type and lavender (Ross,Cochran & Smyth, 1964) stocks of P. americana were used in graft exchanges.Experimental animals were photographed using a Zeiss Tessovar Photomacro-graphic Zoom system. For the SEM, either newly cast exuviae or heads preparedby immersion in liquid nitrogen followed by freeze-drying were mounted onaluminium stubs, coated with a 1-3 nm layer of gold using an ISI sputtercoating unit, and examined with an ISI-60 SEM.

Cockroach eye development 331

Fig. 1. Several types of operations exchanging grafts of eye margin and headepidermis between wild-type and lavender P. americana nymphs were performed todistinguish whether the shape of the vertex epidermis or merely the extent of eyemargin determines the shape of the graft-derived retinal material in the resultingmosaic eye. Following triangular grafts (a), a process of recruitment of vertexepidermis into the growing eye would be detected by widely diverging lateralgraft/host borders. If the eye expands through the proliferation of cells in thebudding zone of the eye margin, mosaic eyes resulting from triangular grafts wouldbe indistinguishable from those resulting from rectangular grafts (b).

RESULTS

Transplantation experiments(a) Triangular grafts

The grafts in this series of experiments had the shape of an isosceles trianglewith an apical angle of 80-90°. The apex of the triangle included a smallamount of eye margin tissue; the rest of the grafted tissue was vertex epidermisfrom the right side of the head (Fig. 1 a). An implant site was prepared byexcising a triangle from the corresponding position in the host animal. Of 50operations, 37 animals showed patches of eye tissue derived from the graft.According to the mode of eye growth, two different results are expected.Assuming that the eye grows by recruitment of vertex epidermis, the triangulargraft tissue would be progressively transformed into eye. During post-operativeeye growth, the shape of the graft-derived retina would be similar to the shapeof the original implant. Tf the eye grows without recruiting epidermis cells, theshape of the graft-derived eye tissue would not be dependent upon the shapeof the implanted vertex tissue. Its shape would depend upon the rate of celldivision within the implanted eye margin only, and on the subsequent growthof the ommatidia.

The shapes of the graft-derived tissue were examined in each of the 37 mosaiceyes. The pattern of eye growth revealed was somewhat variable, but in no caseprovided support for the recruitment hypothesis. In some cases the pheno-typically donor eye tissue appears as an elongated narrow strip often with theoldest graft/host border retaining the original angular profile (Fig. 2). The longedges of such grafts are more or less parallel (Fig. 3). In some cases the longedges do diverge as they approach the eye margin (Fig. 4), but in these cases theincluded angle is much less than the angle of the original graft.

332 MARK S. NOWEL AND PETER M. J. SHELTON

Cockroach eye development 333

If no recruitment occurs, similar results would be obtained whatever theshape of the vertex component of the graft. Consequently, the results of thetriangular graft experiments were compared with the results of experiments inwhich the graft was rectangular in shape.

(b) Rectangular grafts

Of 12 nymphs receiving rectangular implants of donor material (Fig. 1 b),10 were successful. The shapes of the patches of graft-derived ommatidia wereessentially the same as those described above for triangular grafts. That is, theyhad near-parallel long edges which occasionally diverged slightly as theyapproached the eye margin (Fig. 5). It is concluded that the shape of the vertexcomponent of the graft does not affect the shape of the graft-derived eye tissue.That must depend upon the length of eye margin originally implanted.

(c) Transplants of eye margin material only

In these experiments involving 20 animals, a length of eye margin materialonly (Fig. 6) was removed and replaced by a similar genetically marked graftfrom a suitable donor. (The stocks used were wild type and lavender, as before.)Particular care was taken to ensure that no vertex tissue was included in thegraft. The transplanted tissue measured approximately 0-04 by 0-4 mm in arepresentative animal where the graft was measured. A graft of this sizerepresented approximately one half of the eye margin along the dorsal borderof the retina. Because of the size of the graft, this type of transplant is on thelimits of technical feasibility and only one was successful. In this animal, thegraft-derived tissue extended right up to the eye margin (Fig. 8). If recruitmentof head-capsule epidermis had occurred, the graft-derived ommatidia wouldhave been isolated from the eye margin by ommatidia derived from host tissue.Although it would not be worth reporting this single observation by itself, thefact that it is consistent with the other results lends support to the budding-zone hypothesis.

FIGURES 2-5

Figs. 2-4. Mosaic eyes of P. americana generated following transplantations oftriangular grafts as in Fig. 1 a. The angular profile at the base of the graft-derivedommatidia (g) in Fig. 2 is suggestive of the original apex angle (80-90°) of thegrafted integument. The parallel (Figs. 2, 3) or near-parallel (Fig. 4) sides of thegraft-derived retina implies that the non-eye-margin portion (i.e. the head-epidermisportion) of the graft did not become retina.

Fig. 5. Mosaic eye of P. americana generated following transplantation of rectangulargrafts as in Fig. 1 (b). The shape of the graft-derived retina is essentially the sameas that in mosaic eyes following triangular graft implantations (Figs. 2-4). A,anterior; D, dorsal; h, host ommatidia. Bars represent 0-25 mm.

EMB 60

334 MARK S. NOWEL AND PETER M. J. SHELTON

Fig. 6. Portions of the eye margin material only were exchanged between lavenderand wild-type nymphs to determine whether the eye margin acts as an organ forrecruiting adjacent head-capsule epidermis or as a budding zone in the productionof new ommatidia.

{d) Implants of eye tissue into the eye region after retinectomy

One eye was removed from each of 20 newly moulted lavender animals.Great care was taken to excise completely all eye tissue including the eye margin.After one or two moults, the animals were examined for signs of eye regeneration.No regeneration was observed. Exchange transplants were subsequently per-formed using these retinectomized lavender nymphs and wild-type donors. Asection of the wild-type eye was removed and placed in a site on the retinec-tomized lavender host in the position normally occupied by the eye. The graftwas carefully excised with regard to shape and tissue composition. Most of itwas taken from the area of mature ommatidia, but a small portion of the eyemargin and head-capsule epidermis was included (Fig. 7). Thus, host head-capsule epidennis was contiguous with mature retina and eye margin of thegraft. Twelve animals survived the operations and metamorphosed into adults.Of these, six showed signs of the grafted eye tissue. Thus they had one normalunoperated lavender eye and one small graft-derived eye. The experimental eyealways consisted entirely of wild-type tissue. Sometimes the graft formed ajmound of wild-type ommatidia which showed no increase in numbers ofommatidia with successive moults. In three animals this structure's attachmentto the host's head became progressively constricted in subsequent moults, andone was eventually sloughed off. However, in the three other cases the graftformed a growing diminutive eye with a distinct eye margin extending along the

Cockroach eye development 335

Fig. 7. The retina of a lavender P. americana nymph was completely removed. Afterone or two post-operative moults, a piece of regenerated integument from the headcapsule was exchanged with a graft of retina and head epidermis from a wild-typenymph. The purpose of these experiments is to test for recruitment by confrontingwild-type head epidermis, eye margin and mature retina with lavender headepidermis. If head epidermis can be recruited to form retinal tissue, lavenderommatidia will be generated.

dorsal, anterior and ventral borders (Fig. 10). In this type of experiment,lavender ommatidia were never observed in the experimental eye. This meansthat the regenerated eye margin must have formed from the wild-type grafttissue only, and that no head-capsule epidermis was recruited into the eye.

(e) Implantation of regenerated head-capsule epidermis into the eye

As a further test, regenerated head epidermis from the 12 retinectomizedanimals obtained above was implanted into the site left after removal of tissuefrom the wild-type eye (Fig. 7). These operations caused obvious disruptionsin the shape of the growing host retina. The host eye grew around the implanted

336 MARK S. NOWEL AND PETER M. J. SHELTON

ȣ

Fig. 8. Mosaic eye of P. americana resulting from the implantation of a lavendereye margin only into a wild-type host (as in Fig. 6). Note that no wild-type (i.e.host-specific) ommatidia (h) appear subsequent to the establishment of the lavendertissue in the region of the implant, as would be expected if the eye margin acts torecruit the head epidermis.Figs. 9, 10. Compound eyes of adult P. americana following operations as shownin Fig. 7, to confront lavender head epidermis with mature ommatidia and eyemargins of wild-type animals. Figure 9 shows an adult eye after six post-operativemoults following implantation of a graft of lavender head epidermis (g) into awild-type host. The host eye has grown around the lavender epidermis but has notrecruited it, as is demonstrated by the lack of any lavender ommatidia. Figure 10shows a dorsal view of an adult P. americana after six post-operative moults.Following implantation of a graft of wild-type retina into a unilaterally retinec-tomized lavender host, the donor eye (g) has grown considerably in the host, butnot by recruiting adjacent lavender head epidermis, as shown by the lack of anylavender ommatidia. A, anterior; D, dorsal; a, antenna; g, graft-derived tissue; h,host ommatidia. Bars represent 0-25 mm.

Cockroach eye development 337

epidermis which could often be distinguished from the host epidermis by itscolour. However, in none of the 12 animals did any lavender tissue formommatidia (Fig. 9).

In these last two experiments (Wand e), head-capsule epidermis was confrontedwith mature and differentiating regions of the eye, including the eye margin. Ifrecruitment of head-capsule epidermis (by the adjacent retina or eye margin)had occurred, then lavender ommatidia should have formed.

(/) Scanning electron microscopy of the compound eye/head capsule border in G.portentosa

In cockroaches, the head epidermis lying medial to the developing dorsaleye margin contains a small number of bristles. Some are close to the eyemargin. It is possible to count the number of epidermal cells between a bristleand the eye margin by examining the cuticle surface, because the microsculptureof the surface shows a pattern of polygons each representing the area of cuticlesecreted by a single epidermal cell (Hinton & Gibbs, 1971). For this part of thestudy, the cockroach Gromphadorhina portentosa was chosen because the poly-gonal pattern on the surface is much clearer than in P. americana. Since thesame bristles persist from moult to moult (Wigglesworth, 1939; Lees &Waddington, 1942), they can be used as markers to investigate the question ofwhether epidermal cells adjacent to the eye are recruited into it. Thirty newlymoulted G. portentosa and their exuviae (cuticles I) were collected from acrowded stock culture. The head region of each cast exoskeleton was removedand mounted for examination by the SEM. The number of cells betweensuitable bristles and the eye margin was then counted. These 30 animals wereplaced in separate containers and kept until they moulted once more. A secondseries of exuviae (cuticles 2) was then prepared as before. The animals themselveswere killed at this stage by immersion in liquid nitrogen. Their heads werefreeze-dried and prepared for SEM examination. Thus for each animal it ispossible to examine the surface at three stages of growth: in cuticles 1 and 2and in the fixed head (cuticle 3). However, most cockroaches consume the shedexuviae shortly after ecdysis, and for this reason, cuticle 2 was sometimes lost.

Bristles near to the eye margin in cuticle 1 which were recognizable incuticles 2 and 3 were labelled in each series (Fig. 11). Fifty-six bristles in thenine cleanest series were examined in this way. Eighteen bristles (averaging4-2 cells from the eye margin in cuticle 1) show an increase in cell numberbetween the marker bristle and the eye, with an average increase of 1-3 cellsover the two intermoult periods. Six bristles show a decrease during the period(averaging a loss of 1-2 cells from the 5-0 cells separating the bristle from theeye in cuticle 1). Thirty-two bristles separated from the eye by an average of4-1 cells show no change during the three stadia examined. A significantobservation is that in 16 cases, bristles were separated from the eye by onlyone or two cells in cuticle 1 and that this distance was stably maintained incuticles 2 and 3. These data are summarized in Table 1.

338 MARK S. NOWEL AND PETER M. J. SHELTON

(a)

m ^

* *

(b)

Fig. 11. These montages of scanning electron micrographs of a compound eye (ce)of a G. portentosa nymph show that the eye margin does not recruit epidermalcells to supply the growing retina. To show the relationship between the eye marginand the head-capsule epidermis (e) of the growing cockroach, the head cuticle wasexamined at three succeeding stadia. The surface of the retina and the head epidermis(containing bristles) can be seen in cuticle \{a) and cuticle 3(6), nymphal stadiaseparated by two moults. Individual bristles (A-E) are recognizable in these twopreparations by their relative positions. The distance between the eye and thebristles is maintained over the three stadia examined. Bars represent 50/*m.

It is clear from these results that while there is some fluctuation in the widthof the band of cells separating the eye margin from the bristle markers, thereis no trend of a steady decrease in numbers of intervening cells. This trendwould be expected if the eye margin were recruiting the adjacent head epidermis.

DISCUSSION

The preceding results conclusively show that recruitment of non-eye tissuedoes not occur in the postembryonic development of the cockroach eye. LikeBodenstein (1950, 1953) we regard the eye margin as a persistent primitiveportion of the embryonic eye anlage.

Similar conclusions can be drawn from an earlier study. Lew (1933) has

Cockroach eye development

Table 1. Number of epidermal cells between bristle and eye marginin different stadia

339

Retina

A right

E left

M right

Oleft

Qleft

Bristle

abcdhijk

abcdef

abcde

abcde

abcd

Cuticle1

82222331

72

10444

49353

43383

3262

Cuticle3

82232322

83

11244

48354

84393

3282

Change

000+00-+

+++—00

0—00+

++0+0

00+0

Retina

Rleft

R right

Wleft

W right

Bristle

efg

abcef

abcdef

abcdefghi

bcdeg

Cuticle1

266

23

1052

325

1024

24447332

12

39362

Cuticle3

366

23

1053

335

1044

34356322

13

49362

Change

+00

0000+

0+00+0

+0—+—0—0+

+0000

examined the growing dragonfly eye and its relationship with the adjacent head-capsule epidermis. He made tiny wounds in the retina and head capsule of thedragonfly by pricking them with a fine needle. After each moult he comparedthe resulting marks on the exuviae with the scars on the emerged animal andfound that wounds made outside the eye boundary (marked by the buddingzone) never appear in the eye.

The recruitment hypothesis was introduced by Hyde (1972), and results oflater experiments (Lawrence & Shelton, 1975; Green & Lawrence, 1975) wereinterpreted according to her hypothesis. In these later studies, grafts wereexchanged between wild-type and mutant animals with a distinctive eyepigmentation. Typically, a rectangular piece of head epidermis from one

340 MARK S. NOWEL AND PETER M. J. SHELTON

animal was placed in the epidermis of a young host in a site close to thedeveloping and growing eye margin after a similar piece of epidermis had beenexcised from the host. According to Hyde's original (1972) experiments, theimplant was located at a considerable distance from the eye margin andeventually after several moults it was recruited into the eye where it wasrecognizable by its distinctive eye pigmentation. Hyde (1972) also reported thatprothoracic epidermis could be recruited into the eye to form ommatidia.Neither of these claims has been substantiated by subsequent work (Shelton,1976; Shelton, Anderson & Eley, 1977). The latter workers found that veryfew eyes incorporate graft material even when the grafts were made very closeto the eye margin (success rate: 3/50). It occurred to us that this low successrate would be consistent with the occasional inclusion of donor eye margin inthe graft. A similar explanation could also account for the results of experimentson Oncopeltus fasciatus (Shelton & Lawrence, 1974; Lawrence & Shelton, 1975;Green & Lawrence, 1975). Here the animal is so small at the time of theoperation that accuracy in excluding donor eye margin from the graft is evenmore difficult than in P. americana.

Repeating experiments of this sort is of little value. Negative results wouldnot necessarily show that epidermal cells are not recruited for a variety ofreasons. For instance, the graft could be damaged or rejected at the time of theoperation with subsequent migration of epidermal cells into the wound (seeWigglesworth, 1937, 1939). If any cells were subsequently recruited from thesite of the implant, they would have the host phenotype and recruitment wouldbe undetected. Also if the 'eye field' extends only a short distance beyond theeye margin, it is possible that the donor material does not contain any cellswith the competence to form eye if no eye-field cells are included in the graft.In addition, positive results could always be explained in terms of accidentalinclusion of eye-margin tissue in the graft.

The present series of transplant experiments described in this paper avoidsthese ambiguities because eye-margin tissue is intentionally included in thegraft. In conjunction with the observations on bristle position, the presentresults unequivocally demonstrate that the eye margin is a budding zone andthat no recruitment occurs. Since this conclusion contradicts Hyde's (1972)findings, an examination of her experimental approach is necessary.

Hyde's decision to graft between either wild-type or lavender and pearl P.americana partly explains her incorrect conclusions. The pearl mutation resultsin an unpigmented white eye (Ross et al. 1964). However, pearl eyes cansynthesize wild-type pigment when in contact with wild-type or lavender eye-tissue (Hyde, 1972; Shelton et al. 1977). Hyde's criterion that recruitment hadoccurred was the appearance of dark pigmentation within the host pearl eyemargin. However, darkening of the pearl retina can be due to factors other thanthe synthesis of wild-type eye pigments. If damage occurs to eye tissue, asinevitably happens following some of the experimental procedures adopted by

Cockroach eye development 341

Hyde, there would be an influx of haemocytes to the site of damage (Ries, 1933;Wigglesworth, 1937; Ermin, 1939). It has previously been pointed out thatpigmentation in the pearl eye could be due to haemocyte-based melanization(Shelton, 1976). It has also been shown that pearl animals injected with wild-type haemolymph can show a darkening of wounded eyes (Nowel, 1979). Forthese reasons, the appearance of pigment in a. pearl retina does not necessarilymean that they are eye pigments.

Although recruitment does not occur in the postembryonic development ofcompound eyes in hemimetabolous insects, there is a process not unlikerecruitment which has been described for eye development in many holo-metabolous forms. Here the main stages in eye formation occur in the pupawithin a short period of time. In all such forms, retinal differentiation beginson one side of the prospective eye region (Umbach, 1934; Meinertzhagen,1973). This region has been called the differentiation centre, and if it is destroyedby cautery, eye development does not continue (Wolsky, 1949, 1956; White,1963; Wachmann, 1965). In normal development, a wave of developmentalevents spreads out from the differentiation centre in an organizational frontwhich passes across the prospective eye region. In Ephestia ktihnielfa, forexample, a furrow and two waves of mitosis sweep across the prospective eyeregion from the differentiation centre (Egelhaaf et ah 1975; Nardi, 1977).Similar events take place in the eye discs of Drosophila melanogaster (Ready,Hanson & Benzer, 1976). Transplants between eye-colour mutants of E.kiihniella strongly support the notion that the developmental front does actuallypass across the prospective eye region like a travelling wave. The alternativewould be that the developmental front behaves like a standing wave. Accordingto this latter hypothesis, the movement of the wave would be due to theproduction and expansion of ommatidia on one side of the furrow. The resultsof transplant experiments are subject to all the limitations described above,and before it is concluded that the organizational front really does pass acrossthe prospective eye region additional evidence is required. In Drosophila thereis one piece of evidence which would be consistent with a developmental frontpassing across the eye disc. This is that clones generated late in developmentcan cross the eye-head-capsule border (Morata & Lawrence, 1979). If the eyeis formed by the standing wave model, then clones should be restricted to eitherthe eye or the head capsule. Nevertheless, the mechanism in eye developmentin holometabolous insects should not be described as recruitment. Recruitmentimplies that cells which at one time could form head capsule could later beredetermined to form eye. In the case of eye development in holometabolousforms, the front would be passing across tissue which is destined to form eye.To that extent at least, there is no cell transformation and no recruitment.

M.S.N. was supported in part by a Fulbright-Hays Scholarship. P.M.J.S. is grateful toThe Science Research Council for financial support. We thank Dr Ross for supplying ourcultures of lavender and pearl P. americana, and Mr G. McTurk for valuable technicalassistance with the SEM.

342 MARK S. NOWEL AND PETER M. J. SHELTON

REFERENCES

BODENSTEIN, D. (1950). The postembryonic development of Drosophila. In Biology ofDrosophila (ed. M. Demerec), pp. 275-367. New York: Wiley.

BODENSTEIN, D. (1953). Postembryonic development. In Insect Physiology (ed. K. D. Roeder),pp. 822-865. New York: Wiley.

EGELHAAF, A., BERNDT, P. & KUTHE, H. W. (1975). Mitosenverteilung und 3H-Thymidin-Einbau in der proloferierenden Augenanlage von Ephestia kuhniella Zeller. Wilhelm Roux1

Arch. devlBioL 178, 185-202.ERMIN, R. (1939). Uber Bau und Funktion der Lymphocyten bei Insecten. Z. Zellforsch.

mikrosk. Anat. (A). 29, 613-669.FRIZA, F. (1928). Zur Frage der Farbung und Zeichnung des facettierten Insektenauges. Z.

vergl. Physiol. 8, 289-336.GREEN, S. M. & LAWRENCE, P. A. (1975). Recruitment of epidermal cells by the developing

eye of Oncopeltus (Hemiptera). Wilhelm Roux' Arch, devl Biol. 177, 61-65.HINTON, H. E. & GIBBS, D. F. (1971). Diffraction gratings in gyrinid beetles. / . Insect

Physiol. 17, 1023-1035.HYDE, C. A. T. (1972). Regeneration, post-embryonic induction and cellular interaction in

the eye of Periplaneta amerkan. J. Embryol. exp. Morph. 27, 367-379.JORSCHKE, H. (1914). Die Facettenaugen der Orthopteren und Termiten. Z. Wiss. Zool. I l l ,

153-280.KROGH, A. & WEIS-FOGH, T. (1951). The respiratory exchange of the desert locust (Schisto-

cerca gregaria) before, during and after flight. / . exp. Biol. 28, 344-357.LAWRENCE, P. A. & SHELTON, P. M. J. (1975). The determination of polarity in the developing

insect retina. J. Embryol. exp. Morph. 33, 471-486.LEES & WADDINGTON (1942). Development of bristles: Drosophila, Dipt. Proc. R. Soc. Lond.

B 131, 87-110.LEW, G. T. W. (1933). Head characters of the Odonata with special reference to the develop-

ment of the compound eye. Entomologica Am. 14, 41-97.MEINERTZHAGEN, 1. A. (1973). Development of the compound eye and optic lobe of insects.

In Developmental Neurobiology of Arthropods (ed. D. Young), pp. 51-104. London:Cambridge University Press.

MORATA, G. & LAWRENCE, P. A. (1979). Development of the eye-antenna imaginal disc ofDrosophila. DevlBioL 70, 355-371.

NARDI, J. B. (1977). The construction of the insect compound eye: The involvement of celldisplacement and cell surface properties in the positioning of cells. Devi Biol. 61, 287-298.

NOWEL, M. S. (1979). Studies on the developing insect visual system. Ph.D. thesis, Universityof Leicester.

READY, D. F., HANSON, T. E. & BENZER, S. (1976). Development of the Drosophila retina, aneurocrystalline lattice. Devi Biol. 42, 211-221.

RIES, E. (1932). Experimented Symbiosestudien. I. Mycetomtransplantationen. Z. Morph.Oekol. Tiere 25, 184-234.

Ross, M. H., COCHRAN, D. G. & SMYTH, T. (1964). Eye-color mutations in the Americancockroach, Periplaneta americana. Ann. ent. Soc. Am. 57, 790-792.

SHELTON, P. M. J. (1976). The development of the insect compound eye. In Insect Develop-ment-Symposia of the Royal Entomological Society, no. 8 (ed. P. A. Lawrence), pp. 152—169. Blackwell Scientific Publications.

SHELTON, P. M. J., ANDERSON, H. J. & ELEY, S. (1977). Cell lineage and cell determinationin the developing compound eye of the cockroach, Periplaneta americana. J. Embryol. exp.Morph. 39, 235-252.

SHELTON, P. M. J. & LAWRENCE, P. A. (1974). Structure and development of ommatidia inOncopeltus fasciatus. J. Embryol. exp. Morph. 32, 337-353.

UMBACH, W. (1934). Entwiklung und Bau des Komplexauges der mehlmotte Ephestiakuhniella Zeller nebst einigen Bemerkungen uber die Entstehung der optischen Ganglien.Z. Morph. Oekol. Tiere 28, 561-594.

Cockroach eye development 343WACHMANN, E. (1965). Untersuchungen zur Entwicklungsphysiologie des Komplexauges der

Wachsmotte Galleria mellonella L. Wilhelm Roux Arch. EntwMech. Org. 156, 145-183.WHITE, R. H. (1961). Analysis of the development of the compound eye in the mosquito

Aedes aegyptii. J. exp. Zool. 148, 223-240.WHITE, R. H. (1963). Evidence for the existence of a differentiation centre in the developing

eye of the mosquito. / . exp. Zool. 152, 139-148.WIGGLESWORTH, V. B. (1933). The physiology of the cuticle and of ecdysis in Rhodnius

prolixus (Triatomidae, Hemiptera); with special reference to the function of theoenocytes and of the dermal glands, Quart. J. micr. Sci. 76, 269-318.

WIGGLESWORTH, V. B. (1937). Wound healing in an insect (Rhodnius prolixus Hemiptera).J.exp.Biol. 14,364-381.

WIGGLESWORTH, V. B. (1939). The Principles of Insect Physiology. London: Methuen.WOLSKY, A. (1949). The growth and differentiation of retinular cells in the compound eyes

of Bombyx mori. Expl Cell Res. Suppl. 1, 549-554.WOLSKY, A. (1956). The analysis of eye development in insects. Trans. N.Y. Acad. Sci.y

Ser. 11,18,592-596.

{Received 12 March 1980, revised 19 May 1980)