1918] on The Life-History of a Water-Beetle 703 



at the end of which time the skin is cast off and u greenish-white 

 pupa appears. This is more spinose than that of Dijthcm ; but it 

 also prefers to He upon its face, resting upon the two small tail 

 projections and upon the " collar " of the prothorax. 



The perfect insect appears after about ten days, so that the whole 

 life-cycle occupies about nine or ten weeks from the laying uf the 

 egg to the appearance of the perfect insect. This time, however, 

 may be greatly prolonged under less favourable conditions. Thus, 

 the later egg-cocoons produce larvae which take twelve or fourteen 

 weeks to grow up, and the cocoons built in July produce beetles 

 which do not leave the pupal cell for six or seven months. The 

 larvas leave the water in September and even in October, and after 

 three or four weeks turn into pupae. These pupie turn into beetles 

 in late October or November, but the beetles remain, apparently 

 torpid, until the following March or April, when they make their 

 way out and to the water. 



I have mentioned that the larvse of both Dyiiscm and Hydro- 

 charis breathe in the same manner by raising the tail to the surface. 

 The perfect insects, however, assume very different positions when 

 taking in their air-supply. 



Dytiscus floats up to the surface tail tirst, taking in air between 

 the body and the great wing-cases which cover it, and it is in this 

 cavity under the wing-cases that the whole reserve of air is carried. 



On either side of the body under the wing-cases is a row of pits, 

 spiracles ; the last pair of these are much larger than the others. 

 When the insect rises tail first to the surface, the tubes connected 

 with this last pair contract and expand, just as in the larva, renewing 

 the air-supply in the whole tube system, while at the same time the 

 body contracts and expands, renewing the reserve supply under the 

 wing-cases. 



Hydrocharis, on the other hand, comes to the surface head first, 

 turns its head on one side and pushes its short club-like antenna 

 through the surface-film. Now a large part of the under side of this 

 beetle is covered with fine velvety hair, which retains a thin film of 

 air upon it, just as a piece of velvet does when gently pushed under 

 water. When the beetle raises its antenna above the water it brings 

 this film of air into communication with the air above the water. It 

 also has a reserve supply under its wing-cases, and this communicates 

 at the sides with the ventral film, and by expansion and contraction 

 of the body the used-up air is expelled above the water and fresh air is 

 taken in. In Hydrocliaris the most important spiracles are situated 

 well forward, and thus the used air from the air-tubes is expelled and 

 fresh air taken in at the front end of the body instead of the tail end. 



Anyone who examines Hydrocharis and compares it with Dytiscus 

 will at once see great structural differences. In a ventral view of 

 the two types, comparing the heads, the most noticeable difference is 

 in the antenn<3e, which are filamentous in the former and clubbed in 



