Douglas W. Whitman

The significance of body size in the Orthoptera: a review

Abstract This review discusses body size and mass as they relate to the Orthoptera (crickets, katydids, grasshoppers) and the Phasmatodea (walkingsticks). It addresses the expression, causes and consequences of size in these insects. Topics include: methodological problems in body-size research, gravity vs surface forces, allometry and scaling, Dyars law, ontogenetic scaling, size-invariant traits and nonallometric scaling, the influence of size on physiology, function, behavior, life history, mating, fecundity, population dynamics, ecology, and community, size-clines, Bergmanns rule, sexual size dimorphism, Renschs rule, protandry, the environmental, genetic, and physiological control of size, the evolution of size and the influence of size on evolution. Hypotheses are presented to explain why insects remain small in comparison to other taxa.

Sequestration of Glucosinolates by Harlequin Bug Murgantia histrionica

Murgantia histrionica, the harlequin bug, is an aposematic pentatomid that feeds on toxic crucifer plants. By performing predator trials, we found that the bugs are distasteful to several species of bird predators. Given this, we tested the hypothesis that the bugs sequester toxins from the crucifer plants they feed on for use in defense against predation. We used high-pressure liquid chromatography for analyses and tested if M. histrionica sequesters toxic chemicals from its crucifer diet. We found that M. histrionica sequesters mustard oil glycosides, precursors to zootoxic mephitic nitriles, and that sequestration is characteristic of the plant species fed upon. Glucosinolate titers in M. histrionica bodies were 20–30 times higher than in their guts. We found that cabbage-fed M. histrionica had higher titers of cabbage glucosinolates than bugs that were fed on a cabbage diet and then switched to a diet of garden nasturtium. This indicates that M. histrionica immediately sequesters chemicals from whichever plant it feeds upon. The study shows that M. histrionica can sequester glucosinolates from its host plants for use in defense against predation and that the bugs can retain the glucosinolates for an extended period of time.

Chemical Defense in the Stink Bug Cosmopepla bimaculata

Adult Cosmopepla bimaculata discharge a volatile secretion from paired ventral metathoracic glands (MTG) when disturbed. Collected volatiles were similar in both sexes and consisted of n-tridecane (67%), (E)-2-decenal (12%), (E)-2-decenyl acetate (12%), (E)-2-hexenal (3%), hexyl acetate (2%), n-dodecane (2%), a tridecene isomer (1%), and n-undecane, n-tetradecane, and n-pentadecane (all <1%). In addition, undisturbed males produced a novel insect compound, (E)-8-heneicosene, whose function is unknown. The MTG secretion emerges as an enlarging droplet, which is held in place by a cuticular projection and a pleural scent area consisting of specialized rough cuticle surrounding the gland opening. Insects can selectively discharge from either the right or left gland or both glands simultaneously, can control the amount of fluid ejected, and can resorb the ejected secretion droplet back into the gland reservoir. In feeding trials, killdeer (Charadrius vociferous), starlings (Sturnus vulgaris), robins (Turdus migratorius), and anole lizards (Anolis carolinensis) rejected or demonstrated aversion to feeding on the bugs. Furthermore, bugs that lacked the secretion were more susceptible to predation than bugs with secretion, suggesting that the secretion functions in defense against predators.

Gain and loss of multiple functionally related, horizontally transferred genes in the reduced genomes of two microsporidian parasites

Microsporidia of the genus Encephalitozoon are widespread pathogens of animals that harbor the smallest known nuclear genomes. Complete sequences from Encephalitozoon intestinalis (2.3 Mbp) and Encephalitozoon cuniculi (2.9 Mbp) revealed massive gene losses and reduction of intergenic regions as factors leading to their drastically reduced genome size. However, microsporidian genomes also have gained genes through horizontal gene transfers (HGT), a process that could allow the parasites to exploit their hosts more fully. Here, we describe the complete sequences of two intermediate-sized genomes (2.5 Mbp), from Encephalitozoon hellem and Encephalitozoon romaleae. Overall, the E. hellem and E. romaleae genomes are strikingly similar to those of Encephalitozoon cuniculi and Encephalitozoon intestinalis in both form and content. However, in addition to the expected expansions and contractions of known gene families in subtelomeric regions, both species also were found to harbor a number of protein-coding genes that are not found in any other microsporidian. All these genes are functionally related to the metabolism of folate and purines but appear to have originated by several independent HGT events from different eukaryotic and prokaryotic donors. Surprisingly, the genes are all intact in E. hellem, but in E. romaleae those involved in de novo synthesis of folate are all pseudogenes. Overall, these data suggest that a recent common ancestor of E. hellem and E. romaleae assembled a complete metabolic pathway from multiple independent HGT events and that one descendent already is dispensing with much of this new functionality, highlighting the transient nature of transferred genes.

Orientation ofMicroplitis croceipes (Hymenoptera: Braconidae) to green leaf volatiles: Dose-response curves.

FemaleMicroplitis croceipes wasps were tested in a wind tunnel for their ability to orient to various concentrations of eight different green leaf volatile (GLV) substances [hexanal, (E)-2-hexenal, (E)-2-hexen-1-ol, (Z)-3-hexen-1-ol, (E)-2-hexenyl acetate, (Z)-3-hexenyl acetate, (Z)-3-hexenyl propionate, and (Z)-3-hexenyl butyrate]. Overall, the esters elicited the greatest percentage of successful orientation flights, the alcohols elicited an intermediate response, and the aldehydes elicited a low response. The semilog dose-response curves were generally hill-shaped with high responses at medium release rates and low responses at high or low release rates. For the aldehydes, positive responses occurred at all GLV release rates between 0.01 and 100 nl/min. For some alcohols and esters, positive responses occurred at release rates as low as 1 pl/min and as high as 1μl/min. These data show thatM. croceipes wasps are strongly attracted to GLVs and are capable of orienting to GLV concentrations that would occur in nature when a caterpillar feeds on a green leaf. Hence, in nature, GLVs may be important clues, enablingM. croceipes to locate their hosts.

Reduction in diet breadth results in sequestration of plant chemicals and increases efficacy of chemical defense in a generalist grasshopper

The lubber grasshopper,Romalea guttata, is a generalist feeding on a broad diet of many herbaceous plant species and has a metathoracic defensive secretion normally containing phenolics and quinones synthesized by the insect. When insects were reared on a restricted diet of wild onion, they sequestered sulfur volatiles from the plant into their defensive secretions. These compounds were not detected by gas chromatography-mass spectroscopy in secretions of insects on an artificial diet or a natural, generalist diet of 26 plants that included wild onion as a component, nor were they present in secretions from field-collected insects. Defensive secretions of insects reared on wild onion were significantly more deterrent, by as much as an order of magnitude, to two species of ant predators than secretions from insects on either of the other two diets, despite a reduction in the concentration of autogenous defensive chemicals in secretions of insects on the onion diet. Sequestration of plant chemicals that increased defensive efficacy occurred when diet breadth was reduced. We suggest that this occurs because under conditions of specialization, plant secondary metabolites are more likely to be ingested and bioaccumulated in sufficient concentrations to have biological activity against predators. What we define as casual bioaccumulation of bioactive plant chemicals following dietary specialization may lead to evolution of sequestered defense syndromes in insects, and this process may not necessarily require specific adaptation to or coevolution with a toxic host plant.

Morphology and development of oocyte and follicle resorption bodies in the Lubber grasshopper, Romalea microptera (Beauvois)

Abstract We describe the development and appearance of Follicle Resorption Bodies (FRBs) and Oocyte Resorption Bodies (ORBs) in the grasshopper Romalea microptera (= guttata), and demonstrate that these structures can be used to determine the past ovipositional and environmental history of females. In R. microptera, one resorption body is deposited at the base of each ovariole following each gonotropic cycle. These structures are semi-permanent, and remain distinct for at least 8 wks and two additional ovipositions. Ovarioles that ovulate a mature, healthy oocyte, produce a cream-colored FRB. Ovarioles that resorb their oocyte, produce an orange-colored ORB. The number of FRBs equals the number of eggs laid, and the number of ORBs equals the number of oocytes resorbed. Healthy, well-fed R. microptera resorbed about a quarter of their developing oocytes. Starvation or other environmental or physiological stress increased the rate of oocyte resorption and thus increased the number of ORBs. By counting the number and type of resorption bodies in each ovariole, we determined the number of times a female laid, the number of oocytes resorbed, and the clutch size during each oviposition. These data were used to estimate the relative environmental stress experienced by laboratory females during each gonotropic cycle. Oocyte resorption occurred frequently in 1° oocytes, and rarely in 2° oocytes. In healthy, well-fed females, oocyte resorption was temporally staggered, and thus the number of resorbing oocytes increased toward the end of each gonotrophic cycle. Adult virgin females maintained at low density with no male contact, resorbed fewer oocytes than females maintained at higher densities with males, suggesting that high densities or continual male harassment stressed females. In comparison to locusts, R. microptera were relatively slow to initiate oocyte resorption following starvation, requiring >12 d without food before massive resorption was detected.

Predator exaptations and defensive adaptations in evolutionary balance: No defence is perfect

SummaryThe lubber grasshopper,Romalea guttata, is large, aposematic, and extremely toxic. In feeding trials with 21 bird and lizard species, none were able to consume this chemically defended prey. Predators that attempted to eat lubbers, often gagged, regurgitated, and sometimes died. Loggerhead shrikes,Lanius ludovicianus, regularly impale this toxic prey in peninsular Florida. They, like other bird species, are unable to consume fresh lubbers. However, our tests show that they are able to consume lubbers if the prey are allowed to ‘age’ for 1–2 days. This suggests that lubber toxins degrade following death and that shrike impaling behaviour serves as a preadaptation for overcoming the toxic defences of this large and abundant prey. These results also imply that counter adaptations against chemical defences need not involve major morphological or metabolic specializations, but that simple behavioural traits can enable a predator to utilize toxic prey.

A generalist herbivore in a specialist mode Metabolic, sequestrative, and defensive consequences

Adults of a generalist herbivore, the lubber grasshopper,Romalea guttata, can be converted to functional specialists by feeding them exclusively on catnip,Nepeta cataria. No obvious adverse effects on adult development resulted from this enforced monophagy. Notwithstanding the fact thatR. guttata has had no coevolutionary relationship with this Eurasian mint, it readily sequesters compounds that are identical to or derived from the terpenoid lactones that are characteristic ofN. cataria. R. guttata appears to both biomagnify minor allelochemicals and to sequester metabolites of theNepeta terpenes in its paired defensive glands. The levels of autogenously produced phenolics are not affected by feeding onN. cataria and the defensive secretions of catnip-fed grasshoppers are more repellent to ants than those of wild-fed acridids. Metabolites of theN. cataria monoterpenes are sequestered in the defensive glands when catnip is added to the natural diet ofR. guttata. The ability of a generalist,R. guttata, to facilely bioaccumulate a potpourri of foreign allelochemicals when feeding in a specialist mode is analyzed in terms of its biochemical, physiological, and functional significance. Sequestration is examined as a response to the enteric effronteries represented by the phytochemicals that can be characteristic of the “overload” in a monophagous diet.

Large size as an antipredator defense in an insect

Abstract Although large size is considered an evolved antipredator defense for some vertebrates and shellfish, large size is generally not considered an adaptive defensive trait in insects. Here we propose that large size in chemically defended grasshoppers has evolved as a beneficial antipredator trait. The lubber grasshoppers Romalea microptera and Taeniopoda eques are the largest grasshoppers in North America north of Mexico. These closely related species escape most vertebrate predation by possessing powerful predator-deterrent toxins and by nocturnal roosting. We hypothesize that escape from vertebrate predation allowed lubbers to evolve a larger body size, increased fecundity and provided many other benefits, including defense against invertebrate predators. To test the hypotheses that large lubber size reduces predation, we conducted feeding trials with wolf spiders (Honga carolinensis), assassin bugs (Arilus cristatus), preying mantids (Tenodera aridifolia), fire ants (Solenopsis invicta), frogs (Rana pipiens), and birds (Sturnus vulgaris and Passer domesticus). Our results show that larger lubber instars enjoyed a highly significant advantage vis-à-vis predators, demonstrating the adaptive value of large size against both vertebrate and invertebrate predators. Adult lubbers were generally immune from predation. It appears that lubbers have evolved to occupy a relatively predator-free ecological space: they are too large to be attacked by most invertebrate predators and too toxic for most vertebrate predators. We propose an evolutionary scenario whereby a change in feeding behavior toward vertebrate-toxic plants served as an evolutionary breakthrough, setting in motion the subsequent evolution of increased chemical defense and large body size in lubbers. To determine if large size is associated with chemical defense in grasshoppers in general, we compared body sizes of ~ 40 toxic vs ~ 3,000 nontoxic grasshopper species. Our results show that chemically defended species tend to be larger than nondefended grasshoppers, supporting an association between chemical defense and large size in insects. Large size may be favored in insects when vertebrate predation is removed as a strong selective factor.