160 BRITISH TYIiOGLYPHID^. 



inert, and one changed into a Tijroglyplms nymph, 

 leaving its exuvium on the blotting-paper; the other 

 soon afterwards did the same. Early in May one of 

 the nymphs underwent ecdysis and became an adult 

 Tyroglyphus. About 13th May the second became 

 inert, and shortly afterward also became adult. 



The fully-grown nymphs of Tyroglyphus never 

 changed into Hypopi, it was always the young nymph 

 that changed; the change occurs always about the 

 same period of the creature's life-history. It appeared 

 to me, in the species I was observing, that the change 

 occurred at the second ecdysis (first nymphal), and 

 that the form continued until the following ecdysis. 

 By carefully watching known specimens in the small 

 cells I found that all individuals do not become Hypopi 

 in the course of their life-history ; most progress from 

 larva to nymph, pass the nymphal ecdyses, and become 

 images without assuming a hypopial condition; but 

 whatever be the conditions under which the Tyrogly- 

 phidde are kept Hypopi will appear from time to time 

 if the species be one where Hypopi are easy to breed, 

 and if young nymphs be present. 



I found that the Tyroglyphi which emerged from 

 the hypopial skin, when they became adult, were of 

 both sexes ; the female predominated in number, but 

 not more so than it always does in specimens not bred 

 from Hypopus. 



Later on, in 1884, I had ample opportunities of 

 breeding creatures belonging to two other Hypopus- 

 producing genera of Tyroglyphidse,* viz. Histiog aster 

 corticalis and Rhizoglyphus echinopus,a,nd.in both cases 

 I found that the more favourable were the conditions 

 of life, and the more healthful the breeding, the more 

 Hypopi I got. ^ 



What, then, is the reason of the hypopial stage ? It 

 appeared to me to be simply to facilitate the distribution 

 of the species, by enabling the Acarus to lay hold of 



* '' Notes on the Life-bistories of some Little-known Tyroglyphidse," 

 ' J. B. Micros. Soc.,' ser. 2, vol. v (1885), pp. 1932, pi. iii. 



