Modes of defense in nematine sawfly larvae : Efficiency against ants and birds

Ventral glands are common in nematine larvae (Hymenoptera: Symphyta), but they show various degrees of development and are functional for defense only in some species. In those species, volatile irritants are produced which are effective against ants. Alternative or complementary mechanisms against ants are the pubescence ofTrichiocampus spp., the foam pillars constructed byStauronema compressicornis, various movements of the abdomen, which occur independently of the glandular secretion in several species, immobility of the flat larvae ofNematinus luteus, and burrowing within plant tissues in gallicolous larvae or miners. Glandular development is not clearly related to the appearance of the larvae, either cryptic or aposematic. The secretion, even when it is produced in large amounts by species with well-developed glands, is only moderately efficient against great tits. Bright colors are found in gregarious larvae; these were accepted only with reluctance by great tits and sometimes rejected, even species in which the ventral glands are reduced. We suggest that the various volatile irritants secreted by ventral glands are aimed primarily against insects (e.g., ants) and only secondarily against birds.