132 Bulletin of the Brooklyn Entomological Society Vol. XV 



Some larvae, such as the Tipuhdse, draw in the caudal seg- 

 ments by retractor muscles and thus close the spiracles. In 

 Hydrophilus larvae water may enter one tracheal trunk; it is 

 passed through a communicating ramus to the other trunk and 

 ejected, the ejecting mechanism being unknown' (Portier). Still 

 others, and this applies to most larvae, have a spongy network of 

 tissue next to the stigmata, which absorbs any oil and water that 

 happens to enter. Finally, Dytiscus larvae have a slightly altered 

 form of the tracheae which lends itself very readily to absorption 

 of foreign fluids. The tracheal trunks are flattened, a cross- 

 section showing a contour which much resembles that of the 

 human eyelids when widely opened ; the angles attract the foreign 

 liquid and the lumen remains unobstructed. 



(b) Below Surface. — Many surface-breathers are held below 

 surface for more or less prolonged periods, especially during 

 hibernation. We know that a small amount of oxygenation may 

 take place through an air-bubble in the water and also through 

 the plastron of an aquatic insect. But this film of air cannot be 

 likened to a true membrane. Insects held under the surface may 

 capture oxygen bubbles secreted by plants (Wesenberg-Lund, 

 personal observations). In fact, they may do this frequently 

 during the year, even when they can conveniently reach the sur- 

 face. Many beetles do this. Also mosquitoes have been observed 

 thrusting their caudal spiracles into an air-bubble and drawing 

 it into their body. Plastron insects will fuse such a bubble with 

 their plastron. (Parenthetically, mosquito larvae also have caudal 

 blood gills for oxygenation. Let us suppose that some mutant- 

 became a permanent water-breather by means of such gills. 

 What of the present means of mosquito control? What of the 

 future domination of the world?) 



(c) Permanently Belozv Surface. — Some very special adap- 

 tations have been evolved by plastron insects that are found 

 permanently below the surface, or come up comparatively rarely. 

 They are the adult Coleoptera : Elmis, Limnius, and Stenelm^is, 

 of the Parnidae, Hcemonia of the Chrysomelidse, and Hydro- 

 philus. The latter two are also plentiful at the surface. Now 

 let us distinguish at once : Hydrophilus has the underside fitted 

 with hydrofuge hair which helps to hold the external plastron of 



