The secretion of the ventral glands in Hoplocampa sawfly larvae

The defensive secretion of the ventral glands of both species, Hoplocampa testudinea and H. flava is mainly composed of acetogenins. Major compounds are (E)-2-octenal, (E,Z)- and (E,E)-2,4-decadienal and (E,Z,Z)-2,4,7-decatrienal that was identified for the first time in an insect. Benzaldehyde is abundant in H. flava. A comparison of Hoplocampa with taxonomically related genera suggests that, for those acetogenins present as major compounds in the secretion, the chain length decreased during evolution of nematine sawflies. Only the last larval instar of Hoplocampa species produces large amounts of secretions. For H. testudinea, the last instar as compared with previous ones was more efficiently protected against ants in a bioassay, and possessed allometrically larger glands. Hoplocampa larvae live concealed within the fruit that they infest until dropping off the plant for pupation in the soil. The relative importance during the ontogeny of the mechanical protection offered by the fruit and the chemical defence provided by the ventral glands is discussed, also with respect to field observations on the interactions between ants and sawfly larvae. Our hypothesis is that nematine sawfly larvae with a concealed life style will show a relative reduction in glandular size and defensive significance of the glandular secretion. (C) 1997 Elsevier Science Ltd.