Caste developmental pathways in colonies of Coptotermes lacteus (Froggatt) headed by primary reproductives (Isoptera, Rhinotermitidae)

Abstract

Coptotermes is a widespread and economically important genus, but essential features of its caste system remain poorly known. In the Australian species, C. lacteus (Froggatt, 1898), Lenz and Runko (1993) found that orphaned colonies, headed by nymphoid replacement reproductives, produce all year round a brood of nymphs which is almost exclusively male. A detailed study of castes in colonies of C. lacteus headed by primary reproductives was thus desirable, firstly as a fundamental contribution to the understanding of caste evolution in Rhinotermitidae, secondly to provide a sound basis for further studies of sex allocation patterns in this species. After two larval instars without visible differentiation, the development of C. lacteus forks with an apterous line diverging from the nymphal line. The nymphal line comprises 6 nymphal instars followed by the alate. The apterous line includes at least 8, possibly up to 11 worker instars. Among workers of instars 1 – 3, females tend to be sligthtly larger than males, but this sexual dimorphism disappears in older instars. Soldiers are monomorphic and seem to derive exclusively from old female workers. The sex ratio is near 1 : 1 in larvae and young or middle-aged workers, but the oldest worker cohort becomes depleted in females as these moult to presoldiers. Except for the all-femaleness of the soldier caste, the caste pattern of C. lacteus is similar to that of Reticulitermes species and probably close to the ancestral pattern for termites that possess a distinct worker caste.