Larval exocrine glands in the galerucine Agelastica alni L. (Coleoptera: Chrysomelidae): their morphology and possible functions
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Larvae of the alder leaf beetle Agelastica alni (Coleoptera: Chrysomelidae) discharge a fluid from openings located dorsolaterally on paired tubercles of the first to eighth abdominal segments, when disturbed. These paired openings have been described as apertures of defensive glands, whereas the discharged fluid ( = tubercle fluid) is often mentioned as reflex blood. Up to now, clear evidence for the presence of glandular cells below the precisely described apertures was lacking, and detailed knowledge of the composition and defensive efficiency of the discharged fluid was not available. In this study, the internal structures around the paired abdominal openings in A. alni larvae were investigated by scanning electron microscopy and histological methods. Additionally, the composition of tubercle fluid and larval haemolymph was compared by microscopy, tlc, and electrophoresis. The effects of tubercle fluid and haemolymph on ants and conspecific larvae were studied by olfactory and feeding bioassays. ¶The morphological studies revealed that A. alni larvae possess sac-like cuticular invaginations which open dorsolaterally from the spiracle on tubercles of the abdominal segments 1 – 8. A single voluminous gland cell (type III) opens into each invaginated cuticular sac. Each sac may be turned from the inside to the outside and retracted by muscles. The discharged tubercle fluid conforms with haemolymph in its pattern of haemocytes and proteins (SDS-PAGE). Tlc patterns of the tubercle fluid and haemolymph differ by a single spot when using a high polarity eluent. Both tubercle fluid and haemolymph significantly deter the ant Myrmica rubra from feeding. Heating, treating by proteinase K, or exclusion of molecules >3 kD did not inactivate the feeding deterrent quality of tubercle fluid and haemolymph, thus indicating that the feeding deterrents are small, non-volatile, non-proteinous molecules present both in the haemolymph and the tubercle fluid. However, the bioactivity of volatiles from tubercle fluid and haemolymph differed, since the volatiles from tubercle fluid act as alarm pheromone for conspecific larvae, whereas volatiles from the haemolymph do not. The function of the glandular cells associated with the segmental cuticular invaginations of A. alni larvae is discussed.
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