WOODLICE 13 



average of 21| days embryonic and 3 days larval period mPhiloscia 

 muscorum to an average of 64J days embryonic (in first brood) and 

 9 days larval period in Porcellio dilatatus. In these species the breed- 

 ing phase ranges from an average of 36 days in PA. muscorum to 84 

 days in P. dilatatus. The average ratios of the larval period to the 

 embryonic period of development in the brood pouch, in different 

 species, increases approximately in proportion to the average 

 numerical size of the broods. 



The gravidity period for Armadillidium vulgare in Michigan 

 averages 43 days at normal temperatures, according to Hatchett 

 (1947). For Cylisticus convexus it varies from 44 to 62 days and 

 averages 53 days, while Porcellio scaber carries its brood for an 

 average of 44 days. Females of P. (Tracheoniscus) rathkei are gravid 

 on the average for 39 days. Many females of ^. vulgare, C. convexus 

 and P. scaber have at least two broods per year, while P. rathkei 

 usually reproduces only once. The size of the brood in C. convexus 

 ranges from 10 to 70 with an average of 33 and the number of young 

 in broods raised in the laboratory was 24. Under similar condi- 

 tions A. vulgare raised 28, P. scaber 24 and P. rathkei 17 young. 



In contrast in Dallas, Texas, A. vulgare has been found to pro- 

 duce from 29 to 79 young, whilst in Scotland, Collinge (1915) re- 

 ported that individuals raised in captivity had broods of 50 to 150 

 with a maximum in one instance of 267. In France the number 

 varies from 48 to 156. 



When the young are ready to leave the brood pouch they be- 

 come rather active and crawl about until they are released through 

 an opening between the plates of which the pouch is composed. 

 During their departure, the mother usually remains quiet or moves 

 very slowly, keeping the first pair of legs immobile: the young 

 emerge head first and climb down her first or second pair of legs. 

 The majority of the young, after leaving their parent, are not very 

 active and remain together in a group. Towards the end of the 

 emergence period the female moves away, this time using all her 

 legs, and their motion forces the few remaining young from the 

 marsupium. If she is disturbed while carrying young in her brood 

 pouch the female may, by rhythmical contraction of her body, 

 force apart the pairs of plates forming the marsupium and release 

 all her brood at once. A full brood pouch makes it necessary for 



