Reginald F. Chapman

Plant secondary compounds and grasshoppers: beyond plant defenses.

Modern grasshoppers probably evolved from polyphagous ancestors endowed with the ability to tolerate many plant secondary compounds. This tolerance involves various behavioral and anatomical adaptations. Polyphagous grasshoppers have a relatively low level of sensitivity to the taste of many secondary compounds, and, if they do respond to the taste, have the capacity to habituate. This gives time for the induction of detoxifying enzymes so that unpalatable but potentially nutritious plants may be eaten safely. Associative learning involving secondary compounds may be important in food aversion learning, enabling the insects to avoid foods that have inappropriate nutrients, for example. Learning is also involved when grasshoppers develop associations between the taste of chemicals in the surface waxes of plants and internal leaf chemistry, enabling them to make faster decisions about the acceptability of a plant. Anatomically, the midgut ceca of polyphagous grasshoppers have well-developed posterior arms, and it is possible that these are especially important in detoxification, while some species, in addition, have a specialized pocket region in which macromolecules accumulate to be eliminated from the body when the lining of peritrophic envelope is drawn out. Polyphagous species also have thick peritrophic envelopes to which various phenolics become adsorbed. Finally, the midgut environment contains surfactants that reduce tannin–protein complexing except at very high tannin concentrations. Some polyphagous species can utilize secondary compounds as defensive substances or, in one case, in cuticular sclerotization. Grass feeding has evolved on numerous occasions from these polyphagous ancestors, and it has been associated with a loss of many of the characters providing protection from secondary compounds.

Taste sensitivity of insect herbivores to deterrents is greater in specialists than in generalists: a behavioral test of the hypothesis with two closely related caterpillars.

Sensitivity of caterpillars of Heliothis virescens, a generalist, and H. subflexa, a specialist, to eight different plant secondary compounds was examined behaviorally. The compounds were nicotine hydrogen tartrate, hordenine, caffeine, sinigrin, linamarin, arbutin, chlorogenic acid, and salicin. All compounds deterred feeding, at least at the higher concentrations, but the generalist was less affected than the specialist. Thus the hypothesis that specialists have greater sensitivity to deterrents than generalists was supported. In most cases deterrence occurred on first encounter, indicating that the response was sensory; in some cases short-term postingestive effects also appeared to play a role. The larger quantities of deterrent-containing food ingested by H. virescens sometimes resulted in measurable postingestive effects during the second control test. This did not occur in H. subflexa, which more commonly rejected the deterrent-containing food on first contact. The contrast between the species is discussed in relation to tradeoffs involved in different diet breadths.

How female caterpillars accumulate their nutrient reserves.

Female Lepidoptera are often heavier than males. We examined the importance of consumption and post-ingestive processing as mechanisms for female Heliothis virescens larvae to meet the protein and carbohydrate requirements. In experiments in which caterpillars had a choice of diets, enabling them to select an appropriate protein and carbohydrate intake, females caterpillars ate more carbohydrate than males, but only on the heavily carbohydrate biased treatment. Overall, the sexes were not distinguished according to the selective feeding behavior, but females accumulated more protein and carbohydrate over the whole instar than the males did. Additionally, when given no choice, females ate more than males and accumulated more protein provided the diet contained a high proportion of protein. If they were reared on a high carbohydrate diet, there were no differences between the sexes. Our results indicate that female H. virescens larvae accumulate protein by regulating both intake and post-ingestive processing on high protein foods. In the field, late instar H. virescens feed on anthers, which are protein-rich and have the highest amino acid content relative to other cotton floral tissues.

Surviving the flood: plastron respiration in the non-tracheate arthropod Phrynus marginemaculatus (Amblypygi: Arachnida)

Specimens of Phrynus marginemaculatus can remain responsive when submerged in water for more than 24 hours. Behavioral data indicate that P. marginemaculatus utilizes dissolved oxygen from the surrounding water. Scanning electron miscroscopy and light microscope sections show cuticular modifications for plastron respiration. All previous examples of plastron respiration have involved animals with tracheal systems, but amblypygids respire through the use of two pairs of book lungs. This study provides the first example of plastron respiration not only in the order Amblypygi, but also, in any non-tracheate arthropod.

Taste cell responses in the polyphagous arctiid, Grammia geneura: towards a general pattern for caterpillars.

The caterpillars of Grammia geneura are polyphagous as individuals. Electrophysiological responses of its medial and lateral galeal styloconic sensilla to 21 amino acids, 6 carbohydrates, 10 chemically diverse plant secondary compounds and two inorganic salts were examined. In the medial sensillum, a single cell responded to 8 amino acids, 3 carbohydrates, and the iridoid, catalpol, which is present in a favored hostplant. In the lateral sensillum, one cell responded to amino acids and another to fructose. Two cells in each sensillum responded to secondary compounds and it is suggested that the same cells are stimulated by inorganic salts. There was no evidence of a separate salt-sensitive cell. Phenylalanine stimulated a deterrent cell in the medial sensillum and was behaviorally deterrent. Some essential amino acids did not stimulate any cells and it is suggested that a small number of amino acids (sometimes non-essential) may serve as indicators of nutrient quality. Sugars probably serve as the primary phagostimulants because they are in relatively high concentrations in plants. It is proposed that taste receptor cells should be categorized primarily by their behavioral effects as phagostimulatory or deterrent, rather than their specific ranges of responsiveness. This would emphasize the basic similarities across taxa.

Use of cuticular lipids in grasshopper taxonomy: a study of variation in Schistocerca shoshone (Thomas)

Abstract Cuticular lipids of adult Schistocerca shoshone from six different localities in the southwestern United States were analyzed. All the insects had the same hydrocarbons, but their relative abundance varied between populations and, within a locality, remained more or less constant over time. There were some statistically significant differences in the abundance of some compounds in relation to sex and maturity, but the differences were generally small and of doubtful biological significance. Food type had only minor effects on the relative frequencies of compounds, and insects from different populations retained their identities even when reared together in the laboratory. The cuticular lipids of insects from a population living in an area with high summer temperatures included higher proportions of n -alkanes than those of insects from a less extreme environment. If cuticular hydrocarbons are to be used in taxonomic studies of grasshoppers, it is important to examine specimens from a variety of populations and habitats.

A taste receptor neurone dedicated to the perception of pyrrolizidine alkaloids in the medial galeal sensillum of two polyphagous arctiid caterpillars

It is shown that pyrrolizidine alkaloids are phagostimulants for the caterpillars of two polyphagous arctiid caterpillars, Estigmene acrea and Grammia geneura. The caterpillars will also eat dry glass‐fibre discs containing only pyrrolizidine alkaloid − an example of pharmacophagy. The tip‐recording technique is used to demonstrate that each species has a neurone in the medial galeal styloconic taste sensillum responding to pyrrolizidine alkaloids, although the species differ in their sensitivities. This neurone responds to at least four different pyrrolizidine alkaloids and their N‐oxides, and experiments indicate that it is dedicated to perception of these compounds. The sensory response is phasic–tonic and during the tonic phase remains unchanged for at least 500 ms, resembling the type of response often seen in neurones that are sensitive to plant secondary compounds producing deterrent effects.

Electrophysiological studies of olfaction in the whip spider Phrynus parvulus (Arachnida, Amblypygi)

The olfactory response of the whip spider Phrynus parvulus from Costa Rica was examined using a technique analogous to that used for insect electroantennograms on the tarsi of the antenniform legs which bear multiporous sensilla. Responses to 42 chemicals representing different chain lengths of alkanes, carboxylic acids, alcohols, aldehydes, and ketones, as well as some esters, monoterpenes, and phenolics were examined. Fifty-four percent of the chemicals tested elicited responses. Concentration-response curves were generated for guaiacol, hexanal, methyl salicylate, benzaldehyde, octanoic acid, and linalool. Guaiacol, benzaldehyde, and hexanol elicited the greatest responses and no differences were detected between the sexes. Compounds with chain lengths of six carbon atoms generated strong responses and most monocarboxylic acids and ring compounds elicited responses. Some compounds produced increases in potential believed to arise from a hyperpolarizing effect on the neurons. The broad spectrum of chemicals to which these animals respond is similar to results of other studies examining the general olfactory sense of insects. It is possible that odor learning plays a significant role in the behavior of amblypygids.

A neurophysiological study of sensitivity to a feeding deterrent in two sister species of Heliothis with different diet breadths.

Heliothis subflexa and H. virescens are sister species that differ markedly in their hostplant specificity: the former is a specialist on one plant genus, the latter feeds on plants from many families. The behavioral threshold for rejection of deterrent chemicals is lower in larvae of H. subflexa than in those of H. virescens. In this paper, we examine the responses of the galeal styloconic sensilla of these larvae to stimulation by three chemicals, sucrose and inositol, which are phagostimulants, and sinigrin, a deterrent, in an attempt to determine the neural basis for the differences in feeding behavior between the species. The species difference could not be attributed to differences in firing rate of the deterrent-sensitive cells, differences in the ratio of responses to phagostimulants and deterrents, differences in the rates of adaptation of the sensory neurons, or differences in the extent of interactions between chemicals at the peripheral sensilla. We conclude that the differences between the species probably result from differences in processing sensory information within the central nervous system.

The importance of palpation in food selection by a polyphagous grasshopper (Orthoptera: Acrididae)

The behavioral responses of final-instar nymphs of Schistocerca americanato a variety of acceptable and unacceptable plants were recorded. Palpation occurred on all plants and the palps are involved in both acceptance and rejection. On most unacceptable plants, rejection was at first dependent on biting the leaf, but subsequently on Lantana, Machaeranthera, Moms,and Physalis,rejection often occurred after palpation of the surface alone. This is consistent with the suggestion that associative learning occurs. This response did not wane even when the insects had been without food for over 2 h. There was also some evidence of an innate response to the surface characteristics of Physalis.The features of the surfaces that produced these responses were not determined.