172 BRITISH TYROGLYPHEME. 



species, but it did not occur in the life-history of every 

 individual of a species. 



3. That the stage is not the result of desiccation or 

 other unfavourable circumstances, but occurs as often 

 under favourable conditions. 



4. That the stage in the species investigated occupies 

 the period between the penultimate ecdysis and that 

 immediately previous. 



5. That in G. spinipes the Hypopus is fully formed 

 and capable of moving its legs, but not of walking or 

 other active movement; and that it never becomes 

 hard or of the dark colour of ordinary Hypopi. That 

 as a rule it does not ever leave the skin of the young 

 nymph in which it is formed, but that the more adult 

 nymph is formed within the Hypopus, and emerges 

 from it while the Hypopus is still within the young 

 nymphal skin ; so that the Hypopus is not ever seen 

 except that in a few instances it may crawl just out- 

 side the young nymphal skin, when the more advanced 

 nymph is likely soon to emerge. 



6. That in G. domesticus the hypopial stage is even 

 more rudimentary; what represents the Hypopus 

 retaining only the general form, but being without 

 legs or other external organs, and that it never 

 emerges from the young nymphal skin. 



7. That in both species the young nymphal skin 

 within which the Hypopus is developed thickens, and 

 forms a " case " different in appearance from the skin 

 during an ordinary ecdysis. 



While I was engaged on the above-detailed investi- 

 gations Megnin was also observing Glycyphagus, and 

 endeavouring to find a hypopial stage ; he tells us that 

 he failed to find one, but that he discovered something 

 equally curious. He believed that the change of a 

 nymph of Tyroglyphus into a Hypopus was caused by 

 unfavourable circumstances. He says that under similar 

 conditions those of Glycyp/iagus become inert; that a 

 liquefaction of all the organs takes place, " as in a 

 change of skin," and that the gelatinous substance 



