Distribution of photoperiodic receptors in the compound eyes of the bean bug, Riptortus clavatus

  The bean bug, Riptortus clavatus shows a long-day photoperiodic response with respect to the control of adult diapause. The location of photoreceptors for photoperiodism was examined in this species by complete or partial removal of photoreceptor organs. Even after one compound eye or both ocelli were removed, the insects were sensitive to photoperiod. After both compound eyes were removed, however, the insects became reproductive regardless of the photoperiod. Therefore, photoreceptors for photoperiodism were not in the ocelli but in the compound eyes. To clarify whether ommatidia in compound eyes have a regional difference in reception of photoperiod, sensitivity to photoperiod was examined after one compound eye and a part of the contralateral one were removed. Only when the central region of compound eyes was removed did the insects lose sensitivity to photoperiod. It is concluded that the ommatidia in the central region of compound eyes play a principal role in the reception of photoperiod. Accepted: 23 September 1996     

Effects of compound eye-removal on the photoperiodic response of the band-legged ground cricket, Pteronemobius nigrofasciatus

The band-legged ground cricket Pteronemobius nigrofasciatus shows a clear photoperiodic response at 25°C with respect to the control of the induction of embryonic diapause. When crickets were reared under a short-day (LD 12 12) photoperiod and then transferred to a long-day (LD 16 8) photoperiod upon adult emergence, the adults mainly laid nondiapause eggs. However, adults maintained continuously under short-day conditions laid dispause eggs. When compound eyes were bilaterally removed after adult emergence, the crickets mainly laid nondiapause eggs, irrespective of the photoperiod. Thus, the adults completely lost their sensitivity to photoperiod after bilateral removal of their compound eyes. Unilateral removal of the compound eye also affected the crickets under a short-day photoperiod, and the incidence of diapause eggs was intermediate between that laid by intact adults and that laid by adults after the bilateral removal of compound eyes. The incidence of diapause eggs in sham-operated crickets was not significantly different from that in intact crickets under both sets of photoperiodic conditions. These results show that P. nigrofasciatus perceives the photoperiod through its compound eyes.     

Induction of reproductive diapause via perception of photoperiod through the compound eyes in the adult blow fly, Protophormia terraenovae

Covering and surgical removal of the compound eyes were performed to localize photoreceptors for photoperiodic responses in the blow fly Protophormia terraenovae (Diptera, Calliphoridae). Adult females showed a long-day photoperiodic response to control reproductive diapause. When the compound eyes were bilaterally covered with silver paint, diapause incidence increased under diapause-preventing conditions, i.e., a long-day photoperiod and constant light, as though flies were kept under constant darkness. Neither silver painting on a medial region of the head capsule nor control painting in which both compound eyes were painted in a clear solvent caused significant effects on diapause incidence. Unilateral painting of the compound eye caused an increment of diapause incidence under constant light but no effects under a long-day photoperiod. When the compound eyes were bilaterally removed, all the flies developed their ovaries both under a long-day and a short-day photoperiod. Unilateral removal of the compound eye also caused ovarian development under a short-day photoperiod, whereas removal of one antennal lobe or all ocelli caused no effects on diapause incidence. Since P. terraenovae completely lost responsiveness to photoperiod after blinding of the compound eyes, it is likely that this fly perceives photoperiod through its compound eyes. Accepted: 18 February 1997     

Roles of the circadian clock and endocrine regulator in the photoperiodic response of the brown‐winged green bug Plautia stali

The physiological mechanisms underlying photoperiodism in insects have been studied extensively, although the associated molecular machinery remains largely unknown. In the present study, we investigate the roles of the circadian clock gene cycle (cyc) and the endocrine regulator gene myoinhibitory peptide (Mip) in the photoperiodic response of the brown‐winged green bug Plautia stali Scott (Hemiptera, Pentatomidae). Typically, adult females of this species develop their ovaries under long‐day conditions, whereas they suppress its development under short‐day conditions. We find that RNA interference (RNAi) directed against cyc causes malfunction of the circadian clock governing the locomotor activity rhythm and yields abnormal activity profiles not only under constant darkness, but also under light/dark conditions. RNAi directed against cyc and Mip disrupts the photoperiodic response in ovarian development. cyc RNAi suppresses the ovarian development even under long‐day conditions, whereas Mip RNAi induces it even under short‐day conditions. We propose that the core circadian clock gene cyc regulates the photoperiodic response and that Mip is the causal regulator of juvenile hormone biosynthesis in the corpus allatum. Neither photoperiod, nor cyc RNAi affect Mip mRNA levels, and therefore it remains unknown how the photoperiodic information is processed and mediated by Mip.     

Photoperiodic response of larvae of the yellow-spotted longicorn beetle Psacothea hilaris after removal of the stemmata

The role of the stemmata in photoperiodism has been examined in holometabolic insects, but the only reliable results in Coleoptera have been obtained in Leptocarabus kumagaii (Carabidae), the larvae of which do not respond to photoperiod without stemmata. In the present study, photoperiodism was examined in another coleopteran, Psacothea hilaris (Pascoe) (Cerambycidae), after surgical removal of the stemmata. Larvae reared under short-day conditions and transferred to long-day conditions on day 2 of the 5th instar pupated without further larval molts, whereas those continuously reared under short-day conditions underwent supernumerary molts and did not pupate. When the stemmata were removed on day 2 of the 5th instar, the larvae pupated under long-day conditions but did not do so under short-day conditions. However, under long-day conditions some underwent supernumerary molts before pupation. Larvae from which the sensilla trichodeum were removed showed a similar response to that of stemmata-deficient larvae, and larvae from which stemmata were removed at a younger stage (day 2 of the 4th instar) responded to photoperiod similarly to intact larvae. Thus, supernumerary molts under long-day conditions after removal of the stemmata were attributed to injury due to surgery, rather than a change in photoperiodic photoreception. Therefore, we conclude that larvae of P. hilaris show a photoperiodic response after removal of stemmata, in contrast to larvae of L. kumagaii.     

Both the anterior and posterior eyes function as photoreceptors for photoperiodic termination of diapause in the two-spotted spider mite

Photoreceptors involved in photoperiodism in insects and mites can be either the retinal photoreceptors in the visual system or nonvisual extraretinal photoreceptors. Mites with no eyes have a clear photoperiodic response, suggesting the involvement of extraretinal photoreceptors in mite photoperiodism. In mites equipped with eyes, however, it is not known whether the retinal or extraretinal photoreceptors are involved in photoperiodism. The two-spotted spider mite Tetranychus urticae possesses two pairs of eyes. Adult females of this species terminate diapause in response to long days. To investigate whether the eyes function as photoperiodic photoreceptors in T. urticae, their eyes were ablated using a laser ablation system. Mites with their eyes intact terminated diapause under long days after low temperature exposure, whereas they remained in diapause under short days. Under constant darkness, they did not terminate diapause. When all eyes were removed, the mites remained in diapause even when they were maintained under long days. In contrast, the mites showed clear photoperiodic response when only the anterior or posterior eyes were removed. These results indicate that both the anterior and posterior eyes function as photoreceptors in photoperiodic termination of diapause in T. urticae.     

Opsins are involved in nymphal photoperiodic responses in the cricket Modicogryllus siamensis

The cricket Modicogryllus siamensis Chopard shows photoperiod‐dependent changes in the duration of nymphal development: nymphs become adult within 60 days after hatching, undergoing seven moults under long‐day conditions, whereas, under short‐day conditions, nymphal development takes much longer (approximately 180 days) with an increased number of moults. Because removal of the compound eyes alters this photoperiodic response, the eyes may be involved in light detection during the photoperiodic response. The role of opsins, expressed in the compound eye, is examined in the present study with reference to the photoperiodic response. Molecular cloning identifies cDNAs of three opsins, opsin‐Ultra Violet (Ms'op‐UV), opsin‐Blue (Ms'op‐B) and opsin‐Long Wave (Ms'op‐LW), and in situ hybridization reveals that the opsin genes are expressed in specific regions of the compound eye in a gene‐specific manner. RNA interference (RNAi) technology using the opsin genes results in a partial disruption in the long‐day responses; most of the treated crickets showed eight or more moults and up to 23.5% show a prolonged nymphal period that is typical of short‐day responses. Under short‐day conditions, op‐UV RNAi crickets show earlier adult development, whereas no distinct alterations are observed in op‐B and op‐LW RNAi insects. The results suggest that the opsin genes may play differential roles in the photoperiodic response in the cricket and that the results can be at least partially explained in terms of the external coincidence model of photoperiodic time measurement.     

BRAIN PHOTORECEPTOR PATHWAYS CONTRIBUTING TO CIRCADIAN RHYTHMICITY IN CRAYFISH

Freshwater crayfish have three known photoreceptive systems: the compound eyes, extraretinal brain photoreceptors, and caudal photoreceptors. The primary goal of the work described here was to explore the contribution of the brain photoreceptors to circadian locomotory activity and define some of the underlying neural pathways. Immunocytochemical studies of the brain photoreceptors in the parastacid (southern hemisphere) crayfish Cherax destructor reveal their expression of the blue light-sensitive photopigment cryptochrome and the neurotransmitter histamine. The brain photoreceptors project to two small protocerebral neuropils, the brain photoreceptor neuropils (BPNs), where they terminate among fibers expressing the neuropeptide pigment-dispersing hormone (PDH), a signaling molecule in arthropod circadian systems. Comparable pathways are also described in the astacid (northern hemisphere) crayfish Procambarus clarkii. Despite exhibiting markedly different diurnal locomotor activity rhythms, removal of the compound eyes and caudal photoreceptors in both C. destructor and P. clarkii (leaving the brain photoreceptors intact) does not abolish the normal light/dark activity cycle in either species, nor prevent the entrainment of their activity cycles to phase shifts of the light/dark period. These results suggest, therefore, that crayfish brain photoreceptors are sufficient for the entrainment of locomotor activity rhythms to photic stimuli, and that they can act in the absence of the compound eyes and caudal photoreceptors. We also demonstrate that the intensity of PDH expression in the BPNs varies in phase with the locomotor activity rhythm of both crayfish species. Together, these findings suggest that the brain photoreceptor cells can function as extraretinal circadian photoreceptors and that the BPN represents part of an entrainment pathway synchronizing locomotor activity to environmental light/dark cycles, and implicating the neuropeptide PDH in these functions. (Author correspondence: bbeltz@wellesley.edu)     

Compound eyes of insects and crustaceans: Some examples that show there is still a lot of work left to be done

Similarities and differences between the 2 main kinds of compound eye (apposition and superposition) are briefly explained before several promising topics for research on compound eyes are being introduced. Research on the embryology and molecular control of the development of the insect clear‐zone eye with superposition optics is one of the suggestions, because almost all of the developmental work on insect eyes in the past has focused on eyes with apposition optics. Age‐ and habitat‐related ultrastructural studies of the retinal organization are another suggestion and the deer cad Lipoptena cervi, which has an aerial phase during which it is winged followed by a several months long parasitic phase during which it is wingless, is mentioned as a candidate species. Sexual dimorphism expressing itself in many species as a difference in eye structure and function provides another promising field for compound eye researchers and so is a focus on compound eye miniaturization in very small insects, especially those that are aquatic and belong to species, in which clear‐zone eyes are diagnostic or are tiny insects that are not aquatic, but belong to taxa like the Diptera for instance, in which open rather than closed rhabdoms are the rule. Structures like interommatidial hairs and glands as well as corneal microridges are yet another field that could yield interesting results and in the past has received insufficient consideration. Finally, the dearth of information on distance vision and depth perception is mentioned and a plea is made to examine the photic environment inside the foam shelters of spittle bugs, chrysales of pupae and other structures shielding insects and crustaceans.     

How granivorous Coreus marginatus (Heteroptera: Coreidae) recognises its food

Phytophagous insects select their host plant using multiple sensory modalities including vision, olfaction, gustation and mechanoreception. In the laboratory, we investigated the role of vision, olfaction and gustatory sense in the perception of cues released by seeds of different plant species in granivorous Coreus marginatus (Heteroptera: Coreidae). Juvenile insects were put in a choice arena and offered with different seeds on which they feed naturally (Polygonum, Rumex) and seeds which are not their natural food (Stellaria). Individual bugs were manipulated in one of three ways: 1. blackened compound eyes, 2. coated sensillae on antennae, 3. coated tip of labial pouch. Control individuals were not manipulated. The behaviour of each bug was observed for 12 h. Control bugs fed for a significantly longer time than manipulated ones. Individuals with any senses impaired fed on Stellaria, whereas, the control individuals ignored it. The results suggested that seed selection requires information from all three sensory organs: compound eyes, antennae and sensillae on labial pouch. Further research is required to unravel the importance of each sense for the seed selection.     

Opsins and cell fate in the Drosophila Bolwig organ: tricky lessons in homology inference

The Drosophila Bolwig organs are small photoreceptor bundles that facilitate the phototactic behavior of the larva. Comparative literature suggests that these highly reduced visual organs share evolutionary ancestry with the adult compound eye. A recent molecular genetic study produced the first detailed account of the mechanisms controlling differential opsin expression and photoreceptor subtype determination in these enigmatic eyes of the Drosophila larva. Here, the evolutionary implications are examined, taking into account the dynamic diversification of opsin genes and the spatial regulation of opsin homolog expression in other insects. It is concluded that, consistent with their common evolutionary roots, the Drosophila larval and adult eyes use the same mechanisms for the regulation of opsin expression and photoreceptor cell fate specification. Strikingly, the structurally highly derived Bolwig organs retained a more ancestral state of opsin expression and regulation. Inconspicuous in size, the Drosophila larval eyes deliver useful lessons in the reconstruction of homology between neuronal cell types with gene expression data, and on the conservative nature of gene regulatory network evolution during the emergence of novel organs from ancestral templates.     

Time of the season: the effect of host photoperiodism on diapause induction in an insect herbivore,Leptinotarsa decemlineata

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Pigmented eyes,photoreceptor‐like sense organs and central nervous system in the polychaete Scoloplos armiger (Orbiniidae,Annelida) and their phylogenetic importance

The phylogenetic position of Orbiniidae within Annelida is unresolved. Conflicting hypotheses place them either in a basal taxon Scolecida, close to Spionida, or in a basal position in Aciculata. Because Aciculata have a specific type of eye, the photoreceptive organs in the orbiniid Scoloplos armiger were investigated to test these phylogenetic hypotheses. Two different types of prostomial photoreceptor‐like sense organs were found in juveniles and one additional in subadults. In juveniles there are four ciliary photoreceptor‐like phaosomes with unbranched cilia and two pigmented eyes. The paired pigmented eyes lie beside the brain above the circumoesophageal connectives. Each consists of one pigmented cell, one unpigmented supportive cell and three everse rhabdomeric sensory cells with vestigial cilia. During development the number of phaosomes increases considerably and numerous unpigmented sense organs appear consisting of one rhabdomeric photoreceptor cell and one supportive cell. The development and morphology of the pigmented eyes of S. armiger suggest that they represent miniaturized eyes of the phyllodocidan type of adult eye rather than persisting larval eyes resulting in small inverse eyes typical of Scolecida. Moreover, the structure of the brain indicates a loss of the palps. Hence, a closer relationship of Orbiniidae to Phyllodocida is indicated. Due to a still extensive lack of ultrastructural data among polychaetes this conclusion cannot be corroborated by considering the structure of the unpigmented ciliary and rhabdomeric photoreceptor‐like sense organs. J. Morphol., 2009. © 2009 Wiley‐Liss, Inc.     

Existence of an extra-retinal and extra-pineal photoreceptive organ that regulates photoperiodism in gonadal development of an Osmerid teleost, ayu (Plecoglossus altivelis)

Ayu (Plecoglossus altivelis) is an Osmerid teleost whose gonadal development exhibits clear photoperiodism: it is stimulated and prevented under short and long photoperiod, respectively. However, the photoreceptor organ involved in this process remains to be identified. In the present study, we examined whether gonads of ophthalmectomized (Ex) and pinealectomized (Px) ayu respond to short photoperiod to test whether photoreceptors other than lateral eyes and pineal complex are involved in the photoperiodic response of gonadal development. Gonadosomatic index (GSI) and plasma levels of sex steroids (testosterone and 11-ketotestosterone for males and testosterone and estradiol 17-beta for females) were significantly increased in the Ex+Px ayu kept under short photoperiod in both males and females as compared with the initial control. On the other hand, there were no significant increases in GSI and sex steroids in the Ex+Px ayu kept under long photoperiod. Histological observation of gonads in the Ex+Px ayu revealed that oocytes undergoing final maturation in females and proliferation of germ cells in males were observed under short photoperiod but not under long photoperiod. These results indicate that extra-retinal and extra-pineal photoreceptive organ regulates photoperiodic gonadal development in this species.     

A specific area of the compound eye in the cricket Gryllus bimaculatus sends photic information to the circadian pacemaker in the contralateral optic lobe

The circadian locomotor rhythm of the cricket Gryllus bimaculatus is primarily regulated by a pair of interacting optic lobe circadian pacemaker systems. The interaction involves phase-dependent modulation of the free-running period and phase-dependent suppression of activity. Since photic information has been shown to be involved in the interaction, we examined the regional difference in photoreception for the interaction within cricket compound eyes. The activity rhythm of animals receiving partial reduction of one compound eye combined with severance of the contralateral optic nerve split into entrained and free-running components under a 13-h light to 13-h dark cycle. All the animals operated on showed a phase-dependent suppression of activity, and most animals showed a phase-dependent modulation of the period of the free-running component. However, removal of the dorsocaudal area of the compound eye resulted in a severe reduction of the amplitude of the phase-dependent-period modulation. These results suggest that the dorsocaudal portion of the compound eye is a specific region receiving photic signals that are transmitted to the circadian pacemaker in the contralateral optic lobe and that the phase-dependent suppression of activity is caused by a mechanism separate from that for the period modulation.     

Photoperiod perception in the blind mole rat (Spalax ehrenbergi, Nehring): involvement of the Harderian gland, atrophied eyes, and melatonin

The involvement of the Harderian gland, atrophied eyes, and melatonin in the perception of photoperiodic changes has been studied in the mole rat, a fossorial blind mammal the thermoregulatory capacity of which is photoperiod-dependent. When transferred from a long photoperiod to a short one, mole rats increase their resistance to cold, a perfectly reversible phenomenon. After 2 weeks under short photoperiod the thermoregulatory capacities of animals without Harderian glands are less than those of the controls. The Harderian gland appears thus to be implicated in the detection of photoperiodic changes. After 5 weeks, however, the Harderianectomized animals had perfectly integrated the photoperiodic change, so demonstrating that other photoreceptor organs exist. The atrophied eyes, which, under these conditions, do not seem to play an important role, are involved when the animals are transferred from short photoperiod to long photoperiod. Melatonin, but not 5-methoxytryptamine, appears to be a crucial compound in such a phenomenon. These results, which demonstrate that in mammals (at least in the mole rat, as in nonmammalian vertebrates), nonocular photoreceptors exist, suggest that the mechanism by which mammals integrate photoperiodic changes is not the same when the animals are transferred from long to short photoperiod as when transferred from short photoperiod to long photoperiod.     

Electrophysiological responses to light in the compound eyes of some stored-product insects

That stored-product insects demonstrate behavioral response to light is well-known (Stermer, 1959; Sohi, 1966; Yinon & Shulov, 1966). The question asked is whether these insects, which exist always in conditions of low illumination or darkness, possess a normal visual mechanism. In order to clarify this, electroretinograms have been recorded from the compound eyes of the adults of the yellow mealworm beetle Tenebrio molitor L., the hide beetle Dermestes maculatus De G., and the khapra beetle Trogoderma granarium Everts. Nineteen specimens were the subject of experiments in T. molitor and four in each of the other species. Conventional optical and electronic equipment were used to stimulate the eyes and to record the response (Yinon & Aucrbach, 1969). A computer of average transients was used and permitted accurate diagnosis of the response pattern. Fifty responses for each stimulus type (i.e., light intensity) were averaged. A dark adaptation of 5–10 minutes was given before each experiment.