Occurrence and density of Halobates micans (Hemiptera: Gerridae) in the eastern South Indian Ocean

Two species of ocean skaters, Halobates germanus and Halobates micans, live in the tropical and subtropical waters of the Indian Ocean. From December 1992 to December 1993, Halobates was intensively sampled in the easternmost region of the South Indian Ocean (13–18.5°S, 114–121E°), from which there have been a small number of records of Halobates. No H. germanus was caught, but a total of 1190 H. micans were collected, with densities estimated at 13 900–28 100 individuals/km². This suggests that H. micans lives in the study area at high densities comparable to those in the Atlantic and the Pacific Oceans. We also discuss the possible effects of ocean currents and winds on the geographic distributions of the two Halobates species in the eastern South Indian Ocean.     

Transinfection reveals the crucial importance of Wolbachia genotypes in determining the type of reproductive alteration in the host

Wolbachia , a group of endosymbiotic bacteria in arthropods, alter the reproduction of their hosts in various ways. A Wolbachia strain (wSca) naturally infecting the adzuki bean borer moth Ostrinia scapulalis induces male killing, while another strain (wKue) infecting the Mediterranean flour moth Ephestia kuehniella induces cytoplasmic incompatibility (CI) in the resident host. Transinfection of Wolbachia can be a powerful tool to elucidate the relative importance of Wolbachia and the host in determining the type of reproductive alterations. Recently, male killing was shown to occur in E. kuehniella transinfected with w Sca. In the present study, we transferred w Kue to O. scapulalis by embryonic microinjection. In the O. scapulalis transinfected with wKue, CI, but not male killing occurred. Thus, in addition to wSca, wKue was shown to induce the same type of alteration in a foreign host as in its natural host. These results demonstrate the crucial role of the Wolbachia genotype in determining the type of reproductive alteration. However, the present study also revealed the involvement of host factors. First, the degree of incompatibility induced by wKue in O. scapulalis was stronger than that in E. kuehniella , indicating that host factors can affect the level of CI. Second, the vertical transmission rate of wKue in O. scapulalis was generally low, suggesting that the host affects the dynamics of Wolbachia transmission.     

Sexual mosaics induced by tetracycline treatment in the Wolbachia-infected adzuki bean borer, Ostrinia scapulalis.

Wolbachia-infected Ostrinia scapulalis (Lepidoptera: Crambidae) females generate all-female or nearly all-female broods. Curing the infection by tetracycline treatment during larval stages results in the generation of all-male broods in the next generation. Here we show that sexually mosaic offspring are produced by Wolbachia-infected females treated with tetracycline at the adult stage. The sexual mosaics had wings that were composed of distinctive female and male sectors. Besides wings, the sexually dimorphic mid tibiae displayed an intermediate morphology in some of the mosaics. Many of the mosaic individuals had an abnormal structure of the external genitalia as well, a combination of the male uncus and the female ovipositor. We assume that Wolbachia has a feminizing effect on O. scapulalis genetic males and, hence, incomplete curing of the Wolbachia infection results in the generation of sexually mosaic individuals.     

Sex-specific death in the Asian corn borer moth (Ostrinia furnacalis) infected with Wolbachia occurs across larval development.

Maternally inherited endosymbiotic bacteria of the genus Wolbachia induce various kinds of reproductive alterations in their arthropod hosts. In a Wolbachia-infected strain of the adzuki bean borer moth, Ostrinia scapulalis (Lepidoptera: Crambidae), males selectively die during larval development, while females selectively die when Wolbachia are eliminated by antibiotic treatment. We found that naturally occurring Wolbachia in the congener O. furnacalis caused sex-specific lethality similar to that in O. scapulalis. Cytogenetic analyses throughout the entire larval development clarified that the death of males (when infected) and females (when cured) took place mainly during early larval stages. However, some individuals also died after complete formation of larval bodies but before egg hatching, or at late larval stages, even in the penultimate instar. Although the specific timing was highly variable, death of males and females occurred before pupation without exception. The potential association of sex-specific lethality with the sex determination mechanism was also examined and is discussed.     

Upregulation of a desaturase is associated with the enhancement of cold hardiness in the onion maggot, Delia antiqua

Cold-acclimated non-diapause pupae, and summer- and winter-diapause pupae of the onion maggot, Delia antiqua (Diptera: Anthomyiidae), show marked cold hardiness as compared with intact non-diapause pupae. Homeoviscous adaptation of cellular membranes is crucial to enhance the cold hardiness of organisms, and Delta9-acyl-CoA desaturases have been assumed, albeit without experimental evidence in insects, to play a key role in the adaptation. We cloned the cDNA of a desaturase gene (Dadesat) from D. antiqua, which is most likely to encode Delta9-acyl-CoA desaturase. Expression of Dadesat mRNA in the brain, midgut, and Malpighian tubules of cold-acclimated and diapause pupae was upregulated 2-10 fold, correlating well with the increase in cold hardiness. In the pupae with enhanced cold hardiness, palmitoleic and oleic acids, the presumed products of Dadesat, in the phospholipids were significantly increased. These findings suggest that the increase in the expression of Dadesat contributes to enhanced cold hardiness in D. antiqua through the production of these unsaturated fatty acids.     

A method for rearing the yellow-spotted longicorn beetle, <Emphasis Type="Italic">Psacothea hilaris</Emphasis> (Coleoptera: Cerambycidae), to stabilize the last larval instar

The larvae of Psacothea hilaris (Pascoe) grown continuously under a long day in the laboratory pupate after the 4th, 5th, or 6th instar. This developmental polymorphism has complicated studies on the control of metamorphosis in P. hilaris. Since pupation in P. hilaris is known to be suppressed under a short day, a change in the photoperiod from a short to a long day with appropriate timing may assist in obtaining physiologically homogeneous larvae that pupate at the next molt. When the photoperiod was changed from 12-h light:12-h dark to 15-h light:9-h dark at the beginning of the 5th instar, the pupation rate at the next (5th) molt reached 90%, which was significantly higher than the proportion of larvae that pupated after the 5th instar when the larvae were grown continuously under a long day (52%). This rearing technique will expedite studies on the control of metamorphosis in P. hilaris.     

Insights into How Longicorn Beetle Larvae Determine the Timing of Metamorphosis: Starvation-Induced Mechanism Revisited

Larvae of holometabolous insects must determine the timing of their metamorphosis. How they determine this timing has only been studied in detail for a few insect species. In a few species of Coleoptera, starvation is known to be a cue for metamorphosis, leading to the formation of smaller adults (starvation-induced pupation, SiP). We investigated the occurrence of SiP in the beetle Psacothea hilaris. When P. hilaris larvae were starved late in the feeding phase of the last (5th) instar, they exhibited typical SiP characterized by constancy of the time from food deprivation to pupation (TTP) irrespective of the body weight upon food deprivation or the length of prior feeding. In contrast, when larvae were starved early in the feeding phase, TTP decreased by roughly 1 day as the feeding became 1 day longer. The change in the response to starvation was estimated to occur on day 5.9 in the last instar. A series of refeeding experiments suggested that whereas SiP occurred readily in the larvae starved in the late feeding phase, activation of SiP was suspended until day 5.9 in the larvae starved early in the feeding phase. When P. hilaris larvae were fed continuously, they eventually ceased feeding spontaneously and pupated. The time length between spontaneous cessation of feeding and pupation was approximately equal to the TTP in SiP. This suggests that the same mechanism was activated by food deprivation in the late feeding phase and by spontaneous cessation of ad libitum feeding.     

Catch‐up growth in the rhinoceros beetle Trypoxylus dichotomus (Coleoptera: Scarabaeidae): Smaller neonates gain relatively more body mass during larval development

Body size often varies among conspecific neonates. As larger adults generally have higher fitness than smaller conspecifics, it is adaptive for smaller neonates to subsequently gain relatively more size increments during larval development (catch‐up growth). Although catch‐up growth has been suggested in insects, inappropriate methods have been used to examine the size dependence of growth increments. Therefore, it remains unclear to what extent catch‐up growth is common among insects. The present study examined the size dependence of growth increments among larvae of Trypoxylus dichotomus using reduced major axis regression of final to initial body masses. Catch‐up growth was found consistently for larval instars. Furthermore, simulations of the size increments revealed that not only sexual divergence of the mean size, but also catch‐up growth within sexes plays a role in the development of sexual divergence in the body size distribution of T. dichotomus. The significance of catch‐up growth in body size evolution was discussed.