Photoperiodic Control of Diapause Induction in the Larva of Lucilia Caesar

Diapause in insects is herein defined as a state of arrested growth and development which, unlike quiescence, persists until the animal undergoes a certain, well-defined sequence of physiological events that enables development to be resumed. In the green blowfly, Lucilia caesar L., a facultative diapause occurs in the 3rd-instar larva after cessation of feeding but before puparium formation. The ist-instar larva hatches within 24 hr. of oviposition and at 22 C. this stadium lasts for about 24 hr. The 2nd instar also lasts about 24 hr. at this temperature, while the 3rd instar lasts 7-10 days. The first 3 days of the final instar is a feeding period, the remaining 4-7 days being spent as a non-feeding prepupa. As Fraser (1957) points out this is not, strictly speaking, a true prepupa since the prepupal stage is an evanescent 4th instar within the puparium. The non-feeding stage of the 3rd instar, however, has by convention become known as the prepupa. After cessation of feeding, the 3rd-instar larva becomes negatively phototropic, positively geotropic, and under natural conditions burrows into the soil up to a depth of about 6 in. It may then either complete development or enter a prolonged state of diapause, depending upon the season of the year, among other factors. A number of authors (e.g. Roubaud, 1922; Cousin, 1932; Mellanby, 1938; Cragg & Cole, 1952; Fraser, 1957; Fraser & Smith, 1963) have shown that environmental factors such as low temperature, desiccation, lack of aeration and competition for available food act upon the larvae of Lucilia spp. to induce diapause. There is, however, little information available on the effect of photoperiod. Dickson (1949) indicated that this factor did not influence the tendency of L. sericata Mg. larvae to enter diapause, but his results with this species were incidental to the main theme of his research, the induction of diapause in GraphoHtha molesta Busck, and were based on only a few observations. Cragg & Cole (1952) found that the egg-batches from wild L. sericata females caught in late summer produced a high proportion of diapausing larvae even when reared under laboratory conditions favouring normal development. Although they accept that unfavourable environmental conditions acting on the larvae may induce diapause, they concluded that diapause in this species may also be of maternal origin. Fraser & Smith (1963) reached a similar conclusion while maintaining cultures of L. caesar. This paper offers preliminary information concerning the direct effect of photoperiod on diapause induction in larval L. caesar, such information being a prerequisite for the understanding of a maternal influence on larval diapause (the subject of a subsequent communication).