Density-dependent aposematism in the desert locust.

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.

Isolation and structure of a novel charged member of the red-pigment-concentrating hormone-adipokinetic hormone family of peptides isolated from the corpora cardiaca of the blowfly Phormia terraenovae (Diptera).

A hypertrehalosaemic neuropeptide from the corpora cardiaca of the blowfly Phormia terraenovae has been isolated by reversed-phase h.p.l.c., and its primary structure was determined by pulsed-liquid phase sequencing employing Edman chemistry after enzymically deblocking the N-terminal pyroglutamate residue. The C-terminus was also blocked, as indicated by the lack of digestion when the peptide was incubated with carboxypeptidase A. The octapeptide has the sequence pGlu-Leu-Thr-Phe-Ser-Pro-Asp-Trp-NH2 and is clearly defined as a novel member of the RPCH/AKH (red-pigment-concentrating hormone/adipokinetic hormone) family of peptides. It is the first charged member of this family to be found. The synthetic peptide causes an increase in the haemolymph carbohydrate concentration in a dose-dependent fashion in blowflies and therefore is named 'Phormia terraenovae hypertrehalosaemic hormone' (Pht-HrTH). In addition, receptors in the fat-body of the American cockroach (Periplaneta americana) recognize the peptide, resulting in carbohydrate elevation in the blood. However, fat-body receptors of the migratory locust (Locusta migratoria) do not recognize this charged molecule, and thus no lipid mobilization is observed in this species.

The quantitative relationship between life span, food ingestion, egg production, mating, and flight-activity of protein-fed blowfly Phormia terrae novae females.

The age-dependent variations in the amount of food ingestion, egg production, and flight activity of protein-fed female Phormia terrae novae were analyzed. Also the influence of mating and forced physical performance on the time course of these parameters and on life span were tested. In virgin females, the life span with the investigated factors is generally delayed, together with mean and maximum life span. In comparison to mated females virgin females exhibit a 40% prolongation of mean life span and ingest more sugar with a later intake maximum. Their egg production starts 5 days later (Day 10) but reaches only half the amount occurring in mated females. Flight performance is generally higher in virgin females, reaching its maximum 2 days later (Day 14), with a more gradual decline instead of the sudden drop in flight performance seen in mated flies. Females receiving their food by flight only ("forced fliers") show a shorter mean life span (which is more pronounced in virgin flies). The maximum of sugar intake shifts forward to an earlier point after eclosion, and the amount is reduced by about 30% over the entire life span. Similarly, the egg production rate reaches lower values and tends to have an earlier maximum in comparison to corresponding groups that had access to sugar on the cage floor. The results further support the hypothesis that Phormia exhibits a strictly programmed aging pattern. Its time course depends on reproduction sufficiency and physical performance. Both factors act to shorten life expectation.