HYPOPIDJE.. 249 



CASE sign of sex is distinguishable, because it is notorious that so far 

 from a crowd of parasitic insects selecting the same insect as the 

 host in which to deposit their eggs, they carefully avoid placing 

 them in any insect that has already had eggs placed in it by 

 another ; their instinct in this matter teaching them contrary to 

 the axiom acted on by many human couples, that what is enough 

 for one is not enough for two They must, moreover, leave their 

 host at some time or other, and when they do so, they may, of 

 course, be found elsewhere than on insects. Can they be indi- 

 viduals that have passed their parasitic life inside the insects, and 

 are now enjoying their otium cum dignitate idly on its outside? 

 But how did they get out, having no mandibles ? They could not 

 eat their way out. It is not like the thin shell of a Tyroglyphus, 

 which may be cracked by muscular action. And whence come so 

 many? It is true that although a Tyroglyphus may only hold 

 one, a fly might hold many. 



Yet again, how come one or more species of Hypopus to be 

 found habitually, not on insects, but on ferns? In speculating 

 on this we must remember that we have plenty of instances, 

 among insects, of species of the same genus having different 

 habits. In this very group we have one section of Tyroglyphi 

 living on vegetable and another on animal matters — we have 

 most of the species of Glyciphagus feeding on sugared fruits, but 

 other species parasitic \ and it adds to the probability of its being 

 so here that there have been two species of Hypopus found on 

 ferns, and that they both, although distinct, have the same 

 cachet or type of make. They belong to a separate sub-genus of 

 Hypopus. The same remark applies to the specially endowed 

 species found on the field-mouse. It may have special habits 

 as well as special structure. 



Passing from the consideration of this very curious and puzzling 

 animal, let us now glance at some of the different species of the 

 genus. 



