MORPHOLOGY,HISTOLOGY, AND FINE STRUCTURE Fine Structure of the Compound of Callitettix versicolor (Insecta: Hemiptera)

1,2 1,3 LEI-PO JIA AND AI-PING LIANG

Ann. Entomol. Soc. Am. 108(3): 316–324 (2015); DOI: 10.1093/aesa/sav007 ABSTRACT The external morphology and internal anatomy of the compound eyes of the adults of the rice spittlebug Callitettix versicolor (F., 1794) (Insecta: Hemiptera: Cercopidae) are described for the first time with light, scanning, and transmission electron microscopy observations. The eyes of C. versicolor are of the apposition type and each consists of 2,042 ommatidia. Each is composed of a plano-convex corneal lens, a eucone-type crystalline cone, eight retinular cells, two pri- mary pigment cells, and an undetermined number of secondary pigment cells (probably six). The rhab- domeres of the eight retinular cells form a centrally fused rhabdom surrounded by palisades. The distal end of the rhabdom is funnel shaped around the proximal region of the crystalline cone. The microvilli of the rhabdom are arranged in orthogonal directions suggesting polarization sensitivity. With a small F-number of 1.48 and a large interommatidial angle Du of 7.7, the eye possesses high light sensitivity but low spatial resolution. With a large ommatidial acceptance angle Dq of 11.6, the eye can merely form a burred image of its surroundings, but the ratio Dq/Du of 1.5 indicates that the eye can provide a relatively good contrast.

KEY WORDS Hemiptera, Callitettix versicolor, , rhabdom, microvilli

Compound eyes play important roles in ’s forag- largest xylemophagous group of with 1,500 ing, homing, prey detection, and other activities. The described species (Liang and Webb 2002). This group basic structural and functional units of the compound of insects is usually called spittlebugs because their lar- eyes are ommatidia. Each ommatidium generally con- vae live in the spit-like bubbles of foam they secrete, sists of a dioptric apparatus, a few primary and second- while their adults inhabit in a terrestrial setting. Pre- ary pigment cells, and retinular cells (Paulus 1979). vious work on the structure of the compound eyes of The morphological characteristics of a compound eye, the spittlebug species is limited to only one study of such as the organization of the retinular cells and the the meadow spittlebug Philaenus spumarius (L.) microvillus orientations, can provide valuable clues on (Keskinen and Meyer-Rochow 2004). the eye’s capability and limitation (Meyer-Rochow and Here, we are dealing with the compound eyes of an- Monalisa 2009). other spittlebug Callitettix versicolor (F., 1794) (Hemi- In Hemiptera, previous studies on the compound ptera: Cercopidae) which, compared with P. spumarius, eyes have focused mainly on the heteropteran species belongs to a different genus. Another difference be- (Schwind 1983, Settembrini 1984, Fischer et al. 2000, tween the two species is that P. spumarius has a world- Reisenman et al. 2002). However, the fine structure of wide distribution (Keskinen and Meyer-Rochow 2004), the eyes of the species in other hemipteran groups, i.e., but C. versicolor is only found in southern China, In- the Sternorrhyncha and the Auchenorrhyncha, is not dia, Myanmar, Thailand, Vietnam, and Malaysia (Met- well documented. To our knowledge, Drosicha steb- calf 1962). The adults of C. versicolor mainly feed on bingi (Sternorrhyncha: Coccoidea) and Philaenus spu- xylem fluid of rice and maize and are considered one marius (Auchenorrhyncha: Cercopoidea) are the only of the most notorious economic pests in these areas two species in these groups that have had their com- (Chen and Liang 2012). Field observations reveal that pound eyes studied (Ramachandran 1963, Keskinen the adults usually stay at the lower (abaxial) surfaces of and Meyer-Rochow 2004). leaves to avoid high-intensity sunlight for most of the The Cercopidae (Hemiptera: Auchenorrhyncha: Cer- day, and feed themselves at dawn (7 to 8 am) and dusk copoidea) is a superfamily of Auchenorrhyncha and the (5 to 7 pm; Lei et al. 1992). Mating activities occur around 5 pm (Lei et al. 1992) or between 4 and 6 pm (Cai and Xu 2001). 1 Key Laboratory of Zoological Systematics and , Institute Successful rearing of this insect species under labo- of Zoology, Chinese Academy of Sciences, 1 Beichen West Road, ratory conditions (Chen and Liang 2012) makes it pos- Chaoyang District, Beijing 100101, P.R. China. sible for advanced behavioral and ecological 2 University of Chinese Academy of Sciences, Chinese Academy of Sciences, 19A Yuquanlu, Beijing 100049, P.R. China. researches. Given that visual capacity and limitation 3 Corresponding author, e-mail: [email protected]. have great influence on the insect’s mating and feeding

VC The Authors 2015. Published by Oxford University Press on behalf of Entomological Society of America. All rights reserved. For Permissions, please email: [email protected] May 2015 JIA AND LIANG:FINE STRUCTURE OF COMPOUND EYES OF C. versicolor 317 behavior, a thoroughly anatomical investigation on the observed with a regular optical objective (10, major visual organ, i.e., the compound eyes of the NA ¼ 0.3) and an oil immersion objective (100, adults of this species before any further studies seems NA ¼ 1.25) under a Nikon eclipse Ni-E muscope sys- necessary. tem (Nikon, Tokyo, Japan). The objectives of the present study are to provide a Statistics. All measurements were performed in detailed description of the external morphology and in- digital photographs and analyzed using the software ternal anatomy of the compound eyes in the adult of C. Image pro plus 6.0. SEM photographs were used to versicolor and give a brief discussion on the potential determine the dorso–ventral and anterior–posterior functions and limitations of the compound eyes of this lengths of the eye, total number of the ommatidia and pest species. the facet diameters. TEM photographs of cross-sec- tions of the eyes were used to determine the rhabdom diameters, the diameters of pigment granules of pri- Materials and Methods mary pigment cell and secondary pigment cell, as well as retinular cells. LM mugraphs of longitudinal sections Insect Specimens. The adult specimens of C. ver- of the ommatidia were used to determine the ommati- sicolor (10 mm in body length) used in this study dial lengths and the radii of the curvature of the facet. were the third or fourth generation of a laboratory col- The curvature radii of corneal facet lens, as show in ony. The adults of this spittlebug were reared on rice Fig. 3A, were calculated based on the coordinate data seedlings and maintained in the controlled laboratory of three homologous points on their surface (Tanaka condition at 27C, a photoperiod of 14:10 (L: D) h, and et al. 2009). 70% relative humidity (Chen and Liang 2012). Scanning Electron Microscopy (SEM). After decapitation, the heads of three adults were dehydrated Results in an alcohol series, critical point-dried with liquid car- bon dioxide (Leica EM CPD300, Leica Microsystems, External Morphological Features of the Wetzlar, Germany), and sputter-coated with gold at Eyes. The adult of C. versicolor possesses a pair of 40 mA current for 100 s (Leica EM SCD050, Leica brown hemispherical compound eyes on both sides of Microsystems, Wetzlar, Germany). The specimens were the head (Fig. 1A). Each eye measures finally observed under a FEI Quanta 450 scanning 927.3 6 75.5 mm in its dorso–ventral direction (eye electron muscope (FEI, Hillsboro, OR), operated at a height) and 746.7 6 16.5 mm in the anterior–posterior voltage of 30 kV. direction (eye width; Fig. 1A; Table 1). Each eye Transmission Electron Microscopy (TEM). The consists of 2,042 6 83 facets (Table 1). Most of the fac- specimens were decapitated and the heads were imme- ets are typical hexagonal in shape (Fig. 1C), but those diately fixed in 2.5% glutaraldehyde buffered in 0.1 M located close to the periphery region of the eye are cacodylate buffer (pH 7.4) during the day in the light- pentagonal (Fig. 1D). The center-to-center distances adapted state for at least 24 h. The specimens were between neighboring facets measure 18.7 6 3.3 mm postfixed in 1% OsO4, also buffered in 0.1 M cacody- (Table 1). Very sparse interfacetal hairs are present late buffer (pH 7.4) for 2 h at room temperature. The between facets and randomly distributed across the samples were then rinsed three times in the same buf- whole compound eye (Fig. 1B). The interfacetal hair fer and dehydrated in a graded series of ethanol. The is needle-shaped and situated between three neighbor- specimens were passed through different acetone/ ing facets (Fig. 1C). Each interfacetal hair measures Epon mixtures (3:1, 1:1, 1:3, pure Epon), embedded in 2.0–3.5 mm in length and 0.7–1.2 mm in diameter pure Epon (Serva, Heidelberg, Germany) and hard- at its base. The surface of the facets is covered with ened at a temperature of 60Cfor3d.Semithinsec- arrays of corneal nipples, which are randomly tions for light muscopy (LM; from cornea to basement arranged and no orderly nipple pattern is observed membrane) were cut on an ultramutome (Leica EM (Fig. 1C and D). UC6þFC6, Wetzlar, Germany) with a glass knife and Internal Anatomical Features of the stained with a 0.5% aqueous solution of toluidine blue. Eyes. General Organization. Each ommatidium is Ultrathin sections ( 65 nm) were cut on an ultramu- composed of two distinct structures: the dioptric appa- tome (Leica EM UC6þFC6, Wetzlar, Germany) with a ratus, consisting of corneal lens and crystalline cone, diamond knife (Diatome, Bienne, Switzerland) and and the photoreceptive layer, made up of retinular cells picked up on uncoated 200-mesh copper grids. The and their rhabdomeres (Fig. 2). A “clear-zone” has not ultrathin sections were then stained with 2% aqueous been found throughout the eye and the crystalline cone uranyl acetate for 15 min and Reynolds’ lead citrate for is in physical contact with the rhabdom and the com- 5 min and observed under a Spirit transmission elec- pound eye of the adult species of C. versicolor, thus, tron muscope (FEI Tecnai Spirit, Hillsboro, USA) or a conforms to the apposition compound eye type. The JEOL-1400 transmission electron muscope (JEOL, length of an ommatidium near the center of the eye is Tokyo, Japan), both operated at 100 kV. 200 mm. Ommatidial lengths decrease to 150 mm Light Microscopy (LM). The semithin sections of towards the dorsal margin of the eye (Table 1). the eyes cut with glass knifes (see section TEM) were Dioptric Apparatus. The dioptric apparatus consists placed on glass slides and mounted with rhamsan gum. of a plano–convex corneal lens (cornea) and a crystal- After the gum dried out, the samples were then line cone (Fig. 3A). 318 ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA Vol. 108, no. 3

Fig. 1. C. versicolor. (A) A female adult. (B) Surface of the compound eye, showing the hexagonal facets. (C) Front view of three hexagonal facets of the compound eye and the interfacetal hair between them. (D) Front view of a pentagonal facet in the periphery region of the eye.

The cornea is nearly flat in its outer side but shows a Table 1. Morphological and histological measurements of the strongly convex inner surface (Figs. 3Aand4A). The compound eyes in C. versicolor radius of curvature of the inner surface of the cornea measures 12.4 6 1.7 mm(Table 1). Corneal thickness is N Range Avg (mean 6 SD) proportional to ommatidial length and varies 10.0–15.5 mm(Table 1). The cornea is laminated and Eye height (lm) 15 827–1002 927.3 6 75.5 the laminations form a series of parabolas which appear Eye width (lm) 15 718–798 746.7 6 16.5 Number of facets 12 1955–2156 2042 6 83 comparatively loose in the distal side and slightly com- Facet diameter (lm) 150 12.9–22.6 18.7 6 3.3 pact in the proximal side (Fig. 4A, inset). Ommatidial length 20 149.7–201.7 176.6 6 13.1 The crystalline cone, measuring 26.8 6 1.5 mmin (lm) length, lies just beneath the cornea (Fig. 3A; Table 1). Thickness of 32 10.0–15.5 13.2 6 1.2 corneal lens (lm) Formed by four pigment-free cone cells, the cone is of Inner lens radius 15 9.7–14.7 12.4 6 1.7 the eucone type. The nuclei of the cone cells are Crystalline cone 20 24.7–29.4 26.8 6 1.5 packed in the space between the cornea and the crys- length (lm) talline cone (Fig. 3A). In longitudinal section, these Rhabdom length (lm) 20 93.8–130.2 114.3 6 14.3 2 Rhabdom diameter 22 2.6–3.6 3.2 6 0.3 nuclei are oval in shape and measure 7.0 5.0 mm (lm) along their major axes. Except the nuclei, other organ- Microvillus diameter 100 41–69 55.7 6 6.5 elles in the cone cells are undetectable. The cone cells (nm) tapers proximally and form four cone cell roots, i.e., Eye radius (lm) 3 233.1–253.0 244.4 6 10.2 cone cell extensions that occupy small gaps close to the The number of samples (N), the range, and the average are given in rhabdom between adjacent retinular cells in the rhab- each measurement. domal layer (Fig. 5). In longitudinal section, the cone shows a conical shape (Figs. 3Aand4A). In transverse section, the cone displays a circular outline and a peripheral May 2015 JIA AND LIANG:FINE STRUCTURE OF COMPOUND EYES OF C. versicolor 319

An undetermined number (probably six) of secon- dary pigment cells surround the primary pigment cells in each ommatidium. Unlike primary pigment cells, the secondary pigment cells reach from the cor- nea down to the basement membrane and fill the space between adjacent ommatidia. The spherical nuclei of the secondary pigment cells, measuring 2.5 mmin diameter, are present around the central cone (Figs. 3B and 4B). The pigment granules in secondary pigment cells are as large as those found in primary pigment cells. Retinular Cells and Rhabdom. Each ommatidium contains eight retinular cells. They lie just beneath the cone cells and their rhabdomeres form a centrally fused rhabdom. The oval nuclei of the retinular cells measure 4.7 3.4 mm2 along their major axes (Fig. 5). The positions of these nuclei of retinular cells extend from the distal to the middle parts of the rhabdomal layer. Except the prominent nuclei, the cytoplasm of the retinular cells also contains a large number of mito- chondria, many multivesicular bodies, some rough endoplasmic reticulum, and some randomly scattered screening pigment granules (Fig. 5). These pigment granules are 0.35–0.71 mm in diameter and are conspic- uously smaller than those of the primary and secondary pigment cells. Adjacent retinular cells are connected by desmosomes (Fig. 5) near the edge of the rhabdom. Four cone cell roots are also found near the desmo- somes (Fig. 5). According to the positions of the four cone cell roots, the retinular cell which has cone cell root on both sides is numbered R1, and the remaining retinular cells are numbered R2–R8 counterclockwise. Fig. 2. Ommatidium of C. versicolor. (A) Light micrograph of one ommatidium in longitudinal view. (B) All retinular cells contribute muvilli to the rhabdom, Semischematic drawing of one ommatidium in longitudinal which has an average length of 114.3 6 14.3 mm(Table view. bm, basement membrane; cc, crystalline cone; co, 1). Rhabdom diameter is more or less constant and cornea; ppc, primary pigment cell; rcn, nucleus of retinular measures 3.2 6 0.3 mm(Table 1). In longitudinal sec- cell; rh, rhabdom; spc, secondary pigment cell. tion, the rhabdom is not twisted and surrounded by cis- ternae of the smooth endoplasmic reticulum, known as palisades (Fig. 6A). The pigment-free palisades meas- ure 0.25–0.6 mm in width around the rhabdom. concentric layer of high-electron density (Figs. 3Band The distal-most region of the rhabdom is still 4B), and each cone cell contributes one quadrant of open and funnel shaped, allowing the extension of the cone and consequently a cross-like contact face can cone to penetrate into the rhabdomeres (Fig. 4C). In be seen (Figs. 3Band4B). longitudinal section, the distal part of the rhabdom The cone is tapered proximally and penetrates into extends into a funnel shape around the proximal region the rhabdom layer at least 10 mm(Fig. 4C). In the of the crystalline cone (Fig. 4C). In transverse section, cone/rhabdom overlapping area, the cone is “intruded” the rhabdomeres of four retinular cells appear and by four rhabdomeres in transverse section (Fig. 4D). expand to the center of the cone (Figs. 3C, 4D, As the cone tapers, the sizes of the four rhabdomeres and 4E). increase (Fig. 4E). At the central region of the rhabdom, the cone Primary and Secondary Pigment Cells. There are extension disappears and the rhabdom shows a squar- two primary pigment cells enveloping the crystalline ish cross-section profile (Fig. 6B). The muvilli are cone from the distal end close to the cornea to its prox- arranged in orthogonal directions (Figs. 5 and 6B). imal tip where it meets the rhabdom. The nuclei of the The muvilli of rhabdomeres of R2, R3, R4, and R7 are primary pigment cells, situated at the proximal side of all aligned in the same direction, but at right angles to the crystalline cone, are radially elongated and measure those of R1, R5, R6, and R8 (Figs. 5 and 6B). Each 7.2 2.8 mm2 along their major axes (Figs. 3Band muvillus measures 55.7 6 6.5 nm in diameter (Table 1). 4B). Primary pigment cells contain numerous electron- At the proximal end of the ommatidium, the retinu- dense screening pigment granules measuring lar cells form eight axons as a bundle penetrating the 0.54–1.12 mm in diameter. basement membrane (Fig. 6C). Each axon contains 320 ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA Vol. 108, no. 3

Fig. 3. LM micrographs of different levels of the compound eye in C. versicolor. (A) Longitudinal section of the corneal lens and the crystalline cone. (B) Transverse section of the distal region of the compound eye showing the tetrad of the cone and the nuclei of the secondary pigment cells (red arrows) and the primary pigment cells (black arrows). (C) Transverse section of the distalmost region of rhabdom showing the rhabdomeres that are still apart from each other (red arrowheads). (D) Transverse section of the rhabdom. cc, crystalline cone; ccn, nucleus of cone cell; co, cornea; rh, rhabdom.

numerous neurofilaments, some mitochondria, and the crab Ozius truncatus (Shaw and Stowe 1982). The very scarce pigment granules. advantage of this corneal design for aquatic animals is that the optical power changes slightly when the species emerge from the water into the air (Schwind Discussion 1980, Sanders and Halford 1995). Some nocturnal bees Anatomical Aspects. The compound eye of adult and wasps also feature flattened outer and strongly C. versicolor belongs to the apposition type of eyes. convex inner curvatures, but their diurnal relatives do Apposition eyes are generally equipped in diurnally not have the same characteristics (Greiner et al. 2004, rather than nocturnally active insects because of the Greiner 2006b). Thus, the plano–convex corneal relatively low light sensitivity (Warrant and McIntyre design may improve light sensitivity and represent 1993). However, with proper structural modifications, an optical adaptation for nocturnal vision (Greiner apposition eyes in some nocturnal insects like the heli- 2006a). ctid bee Megalopta genalis (Greiner et al. 2004)or The rhabdom in C. versicolor is fused and extends underwater insects like the diving beetle Agabus japo- distally into a funnel shape embracing the proximal end nicus (Jia and Liang 2014), can be effectively adapted of the crystalline cone. According to Fisher et al. to dim-light environment. (2000), all heteropteran species possess open rhabdom, Anatomically, the eye of C. versicolor is similar to while other hemipteran groups, i.e., the Sternorrhyncha that of its close relative, the meadow spittlebug and the Auchenorrhyncha, possess fused rhabdom. P. spumarius (Keskinen and Meyer-Rochow 2004). A This study clearly endorses the conclusion of Fisher noticeable difference between the two species is that et al. (2000). Although the optical function of funnel- C. versicolor possesses a nearly flat outer corneal sur- shaped rhabdomal design is unknown, it occurs in face but strongly curved inner surface, i.e., plano– many insects such as the locust Locusta migratoria convex cornea, while P. spumarius possesses a biconvex (Wilson et al. 1978), the stick insect Carausius morosus cornea (Keskinen and Meyer-Rochow 2004). Plano– (Meyer-Rochow and Keskinen 2003), the cricket Gryl- convex corneal design generally presents in the com- lus bimaculatus (Sakura et al. 2003), and larvae of some poundeyesofmanyaquaticinsectssuchasthe Mecoptera species (Suzuki and Nagashima 1989, Notonecta (Schwind 1980), and crustacean such as Melzer et al. 1994). May 2015 JIA AND LIANG:FINE STRUCTURE OF COMPOUND EYES OF C. versicolor 321

Fig. 4. TEM micrographs of the crystalline cone of the ommatidia of C. versicolor. (A) Longitudinal section (somewhat oblique) of the laminated cornea and the crystalline cone; inset showing the cornea. (B) Transverse section of the crystalline cone, showing the cross-like contact face confined in a circular outline of the crystalline cone. (C) Longitudinal section of the cone/rhabdom overlapping area, showing the proximal end of the cone and the funnel-shaped rhabdom. (D) and (E) Transverse sections of the cone/rhabdom overlapping area showing the cone surrounded by four rhabdomeres. cc, crystalline cone; ccn, nucleus of the cone cell; co, cornea; pg, pigment granule; ppcn, nucleus of the primary pigment cell; rh, rhabdom; spc, secondary pigment cell; spcn, nucleus of the spc. Scale bars ¼ 5 lminA,2lm in inset of A, 2 lm in B, and 1 lm in C–E.

The muvilli of the rhabdom are arranged in two sets along its optical axe (Fig. 6A), which is of importance orienting at right angles to each other, which suggests in discussing polarization sensitivity because twisted polarization sensitivity. In many insects, a group of rhabdom can greatly decrease polarization sensitivity ommatidia suited at the dorsal rim area of compound (Wehner et al. 1975). All these morphological eyes are specialized in detecting polarized light. As features suggest that C. versicolor could discriminate summarized by Labhart and Meyer (1999), the rhab- linearly polarized light, yet behavioral and electrophy- dom in these dorsal rim area ommatida has common siological studies are needed to determine if it is anatomical characteristics: 1) strict alignment of the the case. muvilli and 2) orthogonal muvillus arrangement. In C. Optical Aspects. To estimate the optical property versicolor, the muvilli are extremely aligned along the and limitation of the eyes of C. versicolor, the following rhabdomere and arranged in orthogonal directions parameters were calculated: the F-number, the inter- (Fig. 6C), and thus meet the requirements list above. ommatidial angle Du and the ommatidial acceptance The rhabdom in C. versicolor is also nontwisted angle Dq. 322 ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA Vol. 108, no. 3

Fig. 5. TEM micrograph of the central region of the rhabdom of the ommatidia of C. versicolor showing the fused rhabdom formed by eight retinular cells; black arrows showing the four cone cell roots; circles showing the desmosomes that connect two adjacent retinular cells. mt, mitochondrion; mv, multivesicular bodies; Pa, palisade; pg, pigment granules; R1–R8, retinular cells 1–8; rcn, nucleus of retinular cell. Scale bar ¼ 2 lm.

The focal length f of the ommatidia is determined as Schwarz et al. (2011) (Table 1), the eye of C. versicolor follows (Dore et al. 2005, Meyer-rochow 2007): has a large interommatidial angle Du of 7.7, which suggests the eye has low spatial resolution. By f ¼ðn =n Þcornea surface comparison, most insect predators, such as 0 1 and sphecid wasps, have very small Du of <1 and rhabdom tip distance; thus have high resolution for detecting prey (Land 1997); many common flying insects have Du in the where the refractive indices for the air n0 ¼ 1; for the range 1–3 ; even nonflying insects like the earwig cone of apposition eye n1 ¼ 1.45 (Meyer-rochow 1972); Zophobas moriso has a Du of 5.9 (Nilsson and the distance between cornea surface and rhabdom tip Ro 1994). is 40 mm; and thus the focal length f of the ommatidia The ommatidial acceptance angle Dq is used to is 27.6 mm. Then the F-number can be calculated as determine the angular sensitivity of compound eyes follows: (Warrant and McIntyre 1993).Thelargerthevaluesof Dq, the poorer the spatial resolution, and the more F number ¼ f =D: blurred the image becomes. Ommatidial acceptance angle Dq is defined as follows: With the facet diameter D ¼ 18.7 mm, the eye of C. versicolor has an F-number of 1.48. This value is Dq ¼ d=f ; much smaller because most diurnal insects with apposi- tion eyes usually have relatively high F-number, >2.1 where d is the diameter of the distal rhabdom tip and f (Warrant and McIntyre 1993). Low F-numbers usually is the focal length. With d is 3.2 mm(Table 1), we find indicate high light sensitivity, which suggests the eye of that the acceptance angle Dq in C. versicolor is 11.6, C. versicolor can deal well with a relatively dim-light which shows that C. versicolor could merely form a environment. considerable blurred image of its surroundings. The Interommatidial angle Du determines the spatial ratio of the acceptance angle Dq to the interommatidial resolution of apposition eyes (Land 1981). The smaller angle Du (Dq/Du)is1.5inC. versicolor, which is less the interommatidial angle Du, the greater the potential than the optimal value of 2, but could provide a rela- resolution one eye can get, and vice versa. Interomma- tively good contrast (Land 1997). tidial angle Du is defined as follows (Land 1997): In conclusion, the adult spittlebug C. versicolor pos- sesses an apposition eye with plano–convex corneal Du ¼ D=R; lens and a fused rhabdom formed by two sets of per- pendicular muvilli, which suggest polarization sensitiv- where D is the facet diameter and R is the eye radius. ity. The small value of F-number and large With R ¼ 244.4 mm measured in the way used by interommatidial angle Du indicate that the eye May 2015 JIA AND LIANG:FINE STRUCTURE OF COMPOUND EYES OF C. versicolor 323

Fig. 6. TEM micrographs of the rhabdom of the ommatidia of C. versicolor. (A) Longitudinal section of the central region of the rhabdom showing that the rhabdom is not twisted and is surrounded by palisades. (B) Transverse section of the central region of the rhabdom showing the rectangle outline of the rhabdom and the orthogonal arrangement of the microvilli. (C) Transverse section of eight axons showing many mitochondria (black arrows). pa, palisade; R1–R8, retinular cells 1–8. Scale bars ¼ 1 lm.

possesses high light sensitivity and low spatial resolu- Chen,X.,andA.P.Liang.2012.Laboratory rearing of Callitet- tion. The eye also has a comparatively large ommatidial tix versicolor (Hemiptera:Cicadomorpha:Cercopidae),with acceptance angle Dq that may result in a considerable descriptions of the immature stages. Ann. Entomol. Soc. Am. blurred image. 105: 664–670. Dore,B.,H.Schiff,andM.Boido.2005.Photomechanical ad- aptation in the eyes of Squilla (Crustacea, Stomato- Acknowledgments poda). Ital. J. Zool. 72: 189–199. Fischer,C.,M.Mahner,and E. Wachmann. 2000. The We thank two anonymous reviewers for their constructive rhabdom structure in the ommatidia of the Heteroptera and thorough comments which helped to improve the manu- (Insecta), and its phylogenetic significance. Zoomorphology script. We are also grateful to Zhang Kui-Yan (Institute of 120: 1–13. Zoology, CAS), Liang Jing-Nan (Institute of Microbiology, Greiner, B. 2006a. Adaptations for nocturnal vision in insect ap- CAS), and Sun-Lei (National Laboratory of Biomacromole- position eyes. Int. Rev. Cytol. 250: 1–46. cules, Institute of Biophysics, CAS) for their help with the Greiner, B. 2006b. Visual adaptations in the night-active wasp assistance of the SEM and TEM preparation. This work was Apoica pallens. J. Comp. Neurol. 495: 255–262. supported by the following sources: the National Basic Greiner,B.,W.A.Ribi,andE.J.Warrant.2004.Retinal and Research Program of China (973 Program; grant optical adaptations for nocturnal vision in the halictid bee 2011CB302102), the National Natural Science Foundation of Megalopta genalis. Cell Tissue Res. 316: 377–390. China (grants 31172128 and 31372249), and the National Sci- Jia, L. P., and A. P. Liang. 2014. An apposition-like compound ence Fund for Fostering Talents in Basic Research (Special eye with a layered rhabdom in the small diving beetle subjects in animal taxonomy, NSFC-J1210002), all awarded Agabus japonicus (Coleoptera, Dytiscidae). J. Morphol. 275: to A.P.L. 1273–1283. Keskinen,E.,andV.B.Meyer-Rochow.2004.Post- References Cited embryonic photoreceptor development and dark/light adaptation in the spittle bug Philaenus spumarius (L.)(Homo- Cai, L., and L. J. Xu. 2001. Biological characters of Callitettix ptera, Cercopidae). Struct. Dev. 33: 405–417. versicolor and Abidama liuensis. J. Anhui Agric. Sci. 29: Labhart, T., and E. P. Meyer. 1999. Detectors for polarized 185–186. skylight in insects: a survey of ommatidial specializations in 324 ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA Vol. 108, no. 3

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