Acrididae

Abstract: The ecological processes underlying locust swarm formation are poorly understood. Locust species exhibit phenotypic plasticity in numerous morphological, physiological and behavioural traits as their population density increases. These density-dependent changes are commonly assumed to be adaptations for migration under heterogeneous environmental conditions. Here we demonstrate that density-dependent nymphal colour change in the desert locust Schistocerca gregaria (Orthoptera: Acrididae) results in warning coloration (aposematism) when the population density increases and locusts consume native, toxic host plants. Fringe-toed lizards (Acanthodactylus dumerili (Lacertidae)) developed aversions to high-density-reared (gregarious-phase) locusts fed Hyoscyamus muticus (Solanaceae). Lizards associated both olfactory and visual cues with locust unpalatability, but only gregarious-phase coloration was an effective visual warning signal. The lizards did not associate low rearing density coloration (solitarious phase) with locust toxicity. Predator learning of density-dependent warning coloration results in a marked decrease in predation on locusts and may directly contribute to outbreaks of this notorious pest.

Abstract: The effect of low host plant nitrogen (N) content on herbivore performance has rarely been studied together with the herbivore's feeding behaviour. We explored this relationship with juvenile Omocestus viridulus (Orthoptera: Acrididae) grasshoppers using fertilized and unfertilized host grasses. Due to lower growth rates, grasshoppers reared on N-poor grasses exhibited slightly prolonged development and smaller adult size, while mortality was similar among the fertilizer treatments. This was found both in the laboratory and in outdoor cages under natural climatic conditions. A parallel analysis of feeding behaviour revealed that the grasshoppers counterbalance N shortage by compensatory feeding, and are capable of selectively feeding among grasses of contrasting nutritional quality when given a choice. This indicates a striking ability of O. viridulus to regulate nutrient intake in the face of imbalanced food sources. Although the species exploits a relatively very poor autotroph nutrient base in the wild, as underpinned by N analysis of natural host grasses and grasshopper tissue, our data suggest that natural food quality imposes no relevant constraint on the herbivore's performance. Our study thus challenges the importance of simple plant-mediated control of herbivore populations, such as N limitation, but supports the view that herbivores balance their intake of N and energy.

Abstract: Numerous investigations in the past by many competent observers of the structure and of the relationship between the various parts of the insect chemoreceptor sensory neurones, trichogen and tormogen cells, cuticle and neurilemma have left us with a number of unsolved problems. The structures are small and extremely difficult to fix and section properly because of the tough and impermeable cuticle of which they are, in part, composed. Even when satisfactory preparations are obtained the highest powers of the light microscope fail to reveal essential details such as, for example, the exact location and character of the junction between the tip of the distal process of the sensory neurone and the cuticle. With the electron microscope it is now possible to add new information to that which we already possess and to support or correct earlier interpretations. In the present study both the light and the electron microscope have been employed. All of the d%culties which are encountered in iiXing and sectioning insect cuticle for study