24 Transactions of the Society. 



4. Hijpopus is the male of Tijroghjphus. 



5. Hypopus is an adult itch-mite. 



6. Hypopus is a ferocious parasite, sometimes external, some- 

 times internal, which ends by entirely devouring its host from 

 within, leaving only the skin. 



7. Hypopial form is a travelling dress. 



8. Hypopus is the cuirassed, heteromorphous, adventitious 

 nymph of Tyroghjphus, &c., appearing only for the distribution and 

 preservation of the species under adverse circumstances. 



This eighth view, which is not inconsistent with the seventh, 

 was that originated by P. Megnin of Versailles, and has been that 

 most ordinarily received of late years. Megnin's opinion was 

 that when the food of the TyrogJyphus became dry, and the 

 atmosphere also too dry for the continued existence of the 

 Tyroglyplius, &c., then the ordinary nymphs of these creatures 

 changed into Hypopi which had the power of supporting greater 

 draught and more unfavourable circumstances than the Tyroglyphi, 

 and of attaching themselves to insects, &c., whereby they got 

 conveyed to new and more favourable situations, when they once 

 more assumed the form of Tyroghjphus nymphs. After a careful 

 series of observations I have elsewhere* given my reasons for 

 considering that, although Megnin was correct in the major part 

 of his contention, yet he was in error in that portion of it which 

 ascribed the production of Hypopus to exceptional adverse circum- 

 stances, and that, in fact, although it was not all the individual 

 nymphs of any species of Tyroglyphus, &c., that became Hypopi, 

 yet the hypopial period takes the place of that between two 

 ecdyses in the ordinary life-histoiy, and that the change to 

 Hypopus is not caused by unfavourable circumstances, and is not 

 any extraordinary or exceptional occurrence, but is a provision of 

 natui-e for the distribution of the species occurring irrespective of 

 adverse conditions. I found a very strong confirmation of my 

 views on the point in the species which I have been dealing with 

 in this paper : they were apparently under circumstances which 

 suited them ; they had ample food which they appeared to feed 

 upon readily ; the moisture was abundant but apparently not in 

 excess ; the adult males and females, and the ordinary nymphs and 

 larvae did not die nor dry up, but on the contrary appeared healthy 

 and well fed, but yet the Hypopi were present among the 

 specimens in their natural locality in the reeds, and kept on ap- 

 pearing in my breeding-cells in considerable numbers. The more 

 ordinary nymphs I had, the more Hypopi I got, and in order to 

 obtain Hypopi I adoj)t precisely the same means as I should use to 

 keep Acari which do not assume the hypopial form in perfect 



* "The Hijpopns Question," Jyiim. Linn. Soc. (Zool.) xvii. (1884) pp. 371-94. 



