Observations on the Life‐Histories of some Terrestrial Isopods

Abstract

Summary. 1 Numerous specimens of six common species of British Woodlice have been kept in captivity and allowed to breed freely for two-and-a-half years. Observations have been made on their habits, choice of food, humidity requirements, moulting, growth stages, and breeding, and compared with parallel observations in the field. 2 The choice of habitat of Woodlice is largely determined by their humidity requirements, which vary considerably with the species. This factor is of greater importance than food supply, light, temperature, or oxygen requirements. 3 The durations of the successive stages in the breeding processes, whilst varying with the different species, are more or less constant for each particular species. There is, however, a universal slight departure from this regularity of the stages among (a) those which are the earliest in the year of each species to breed, (b) those producing a second brood in the year. The regularity in the c which arc connected with the breeding processes is not shown during the non-breeding periods of the year, or in the male sex. 4 The average ratios of the larval period to the embryonic period of development in the brood pouch, in the different species, increase approximately in direct proportion to the average numerical sizes of the broods. 5 The moulting and growth stages of the young of all the species show a marked regularity in their times of occurrence, up to the age of at least six months. Their rate of increase in size is fairly constant, and is not apparently governed by the mounts. 6 A comparative study of the growth of the young shows that certain anatomical features, common to all the species, characterize successive stages in growth, whilst the order in which the distinguishing characters of the different species arise in development coincides with the line of ontogeny tacitly assumed in the classification of the group. 7 The adults show a definite and well-marked seasonal periodicity in their phases of breeding activity, which varies both in time of occurrence and in duration with the different species, but which is more or less constant for each particular species. The times of maximum breeding activity occur twice in one year in some species (in which case only from 10—-30 per cent, of the individuals breed on the second occasion), and only once in other species. 8 A resume is given of the entire life-history, in the course of which attention is directed to a consideration of the controlling factors of certain stages and physiological processes concerned with growth and moulting.