144 BRITISH 



Megnin considered that these immature forms were 

 hypopial ; * they, however, appear to me to be very 

 different from what are usually considered to be 

 hypopi. It is true that the Acarus which Megnin 

 considers to be the hypopial nymph of Pterolichus 

 falciger is stated to be without mouth-organs ; but it 

 lives in the subcutaneous cellular tissue of birds ; and 

 it is a very different thing for an animal in a parasitic 

 stage to be without mouth-organs when it lives almost 

 always bathed in liquid food-material which it can 

 absorb, from what it is to be similarly deficient when 

 it lives as a free temporary external parasite of hard 

 chitinous creatures from which it cannot obtain any 

 nourishment whatever, or as a free non-parasitic 

 creature. At any rate, whether these exceptional and 

 doubtful cases are really to be considered as hypopial 

 or not they will not be treated of here, and I shall deal 

 only with the hypopial stage in the Tyroglyphidse. 



The history of the literature relative to Hypopus, 

 and the errors and gradual steps through which our 

 present knowledge of the stage has been acquired, are 

 interesting. 



In August, 1735, de Geerf noticed, for the first 

 time, on the house-fly (Musca domestica) some small 

 reddish Acari, which were in such numbers that the 

 neck and back of the fly were entirely covered by 

 them. The mites were usually quite still, but ran 

 about actively when touched; the body was oval, 

 entirely chitinised, polished, convex on the dorsal, flat 

 on the ventral surface ; instead of an ordinary mouth 

 it had a minute membranous tube, or what de Greer 

 calls a trumpet, articulated to the ventral surface about 

 where the mouth would be ; this organ was closed 

 distally, and ended in two hairs, often longish. Looked 

 upon in the light of later knowledge this " trumpet " 



* ' Les Parasites et les Maladies parasitaires,' Paris, 1880, p. 151, 

 and some others of Megnin's writings. 



f ' Memoires pour servir a 1'histoire des Insectes,' Stockholm, 1752- 

 78, vol. viii, p. 115, pi. vii, figs. 1 3. 



