552 



PHYSIOLOGY. 



307- 



With respect to their relation to other animals, it is probable that 

 none takes place between them and the lowest in the scale, as the 

 majority of the latter dwell in water, whereas insects are air-animals. 

 The lower animals also are especially found in the sea, which no insect 

 inhabits. The lower fresh-water animals are partly too small to supply 

 insects with sufficient food, and partly again too large to be conquered 

 by the rapacious water insects. Yet I have sometimes observed leeches 

 (Hirudo vulgaris and species of Clepsini) in the power of the large 

 Dylici. We may also admit that these water animals exercise the 

 right of retaliation, and also devour small insects, at least those among 

 them which are appointed to feed upon animal matter ; yet they would 

 doubtlessly offer but little nutriment to the leeches, as these seek the 

 blood of the vertebrata. Nor do insects appear to stand in any very 

 near relation to the Mollusca, particularly as the majority of these 

 inhabit the sea. The larvae of many insects may serve fresh-water 

 snails as food, whereas the land snails, which feed upon vegetable sub- 

 stances, are attacked by many insects which as larvae live parasitically 

 upon them. Mielzinsky first discovered the larva and apterous female 

 of Drilus flavescens in the shells of snails in the vicinity of Geneva, 

 and described it under the generic name of Cochleoctonus. Subse- 

 quently Victor Audouin and Desmarest made further observations 

 upon this parasite, which is not rare upon Helix nemoralis, and 

 explained its development to the perfect insect *. The larvae also of 

 the Lampyri, which are closely allied to the genus Drilus, live parasi- 

 tically in snails, as Audouin has communicated -j-. A third insect, the 

 maggot of a small fly, the perfect state of which, however, is not known, 

 is said to live as a parasite in the body of the garden snail, and even to 

 show itself in its feelers. These are the only instances hitherto known 

 of insects living as parasites upon Molluscu. 



But we find the relation of insects to other articulata, namely, to 

 the Arachnodea and to other insects, much more common. The 

 Crustacea almost all live in the sea, and are therefore secured from the 

 attacks of insects, and no parasitic insects have yet been found upon 

 the Myriapoda and wood lice. The spiders are also free from their 



* Anmdes dcs Sciences Nat,, torn. i. p. 67 ; torn. ii. p. 129 and 443. 

 t lb. torn. vii. p 353. 



