Kentromorphic phases in three species of Phasmatodea.

Abstract

Nymphs of the injurious Australian Phasmatidae Podacanthus wilkinsoni Macl., Didymuria violescens (Leach), and Ctenomorphodes tessulata (Gray) were reared from the early instars in crowds and in isolation. Typical kentromorphic phase differences, quite analogous to those of locusts, developed in the colour pattern of the isolated and crowded insects and persisted into the adult stage. This is believed to be the first record of kentromorphic phases in the Phasmatodea. The two types of pattern are described and illustrated in colour. The "lowdensity phase" is rather uniform and usually green, while the "high-density phase" is conspicuously patterned with black, yellow, and sometimes white. The extreme conditions are connected by all intergrades and for Podacanthus a rating scale has been used to measure the phase level attained by individual insects. Observations were made on the mean phase rating corresponding to different densities of Podacanthus nymphs on eucalypt saplings in the field. The rating rose from about 1.5 at densities of one insect per 10-20 15-in. branchlets to the scale maximum of 4.0 at densities of about one insect per branchlet. A correlation was also found between the ratings of museum specimens and subjective estimates of abundance recorded by their collectors. All three species showed morphometric phase differences also analogous to those of locusts: the length of femur III, the ratio length of femur III/width of head, and the sexual dimorphism with respect to width of head all tended to lower values in the high-density phase. The phasmatids showed no overt gregariousness and Podacanthus nymphs spent 95 per cent, of their time at rest, even in crowds of the high-density phase. Thus, in this group, activity and gregariousness can play no part in phase change. It is therefore probable that, both here and in locusts, the physiological mechanism of pattern change involves a direct nervous link between sensory stimulation and the endocrine system responsible for the pigmentation. Neither Uvarov's nor Kennedy's theory regarding the biological function of phase pattern change is applicable to the Phasmatidae and it must be doubted that either is applicable to locusts. It is suggested that the colour-pattern aspect of kentromorphic variation may be a mechanism for effecting a switch-over from procryptic to aposematic adaptations in nice correlation with the population densities under which each of these has survival value.