INSECTS 



29 



strong hairs on the inner side. In swim- 

 ming the stroke is made by both legs at 

 once. Perhaps the most interesting facts 

 about these beetles are those associated 

 with their method of breathing. 



The horny wing cases covering the ab- 

 domen are very thick and fit close against 

 the abdomen^rcxcept at the extreme pos- 

 terior end of the body. The space be- 

 tween the wing cases and the upper sur- 

 face of the abdomen forms a large air 

 space. The spiracles, or openings into 

 the respiratory system, are situated at the 

 margins of the upper side of the abdo- 

 men. When the beetle comes to the sur- 

 face for a fresh supply of air it exposes 

 the tip of the body and then by a depres- 

 sion of the tip of the abdomen allows a 

 fresh supply of air to enter into the cav- 

 ity below the wing covers ; this cavity is 

 then closed and the beetle is ready for 

 another trip under the water. When rest- 

 ing in the water they float with their head 

 downward and the end of the abdomen 

 slightly projecting from the water; thus 

 a fresh supply of air is easy to secure. In 

 their food habits these beetles are preda- 

 ceous, and in addition to other insects, 

 will even kill small fish. 



The Water-scavenger beetles are not 

 such perfect swimmers as the Predaceous 

 ones. When the latter makes a stroke in 

 swimming it strikes with both hind legs, 

 while the Scavengers strike alternatingly 

 with the hind legs. Their method of se- 

 curing and carrying air, as with other 

 water-beetles, is remarkable. In addition 

 to the air reservoir under the wings, they 

 have on the under side of the body large 

 hairy areas which communicate with the 

 one under the wings. All the air spaces 

 are thus in direct communication. The 

 respiratory openings in the Predaceous 

 water-beetles open on the upper side of 

 the abdomen, but in these beetles they are 

 on the lower side and surrounded by short 

 hairs which preserve the air film on the 

 lower surface. When the fresh air supply 

 has been exhausted the beetle comes to 

 the surface, tips the body slightly, so as 

 to bring the region on one side of the 

 body just behind the head, to the surface. 

 The long antenna which is folded back- 

 ward and reaches to the rear part of the 

 head, occupies an air space in its apical 

 half, and in addition is covered bv fine 



hairs, thus being doubly protected from 

 being wetted. At the moment the beetle 

 reaches the surface, by a stroke of the 

 antenna (on the side which is nearest the 

 surface, the body being tilted), the film 

 from the air space in which the antenna 

 rests is carried upward and outward to 

 the surface of the water, thus forming an 

 opening to the exterior. By movements 

 of the wings', aided by bellows-like con- 

 tractions and expansions of the body, a 

 fresh supply of air is pumped into the air 

 reservoir. 



In speaking of peculiar water insects 

 one must not forget to mention the larva 

 of Donacia. The adult female of this in- 

 teresting leaf-eating beetle often cuts cir- 

 cular holes in the large leaves of water- 

 lilies, and then deposits her eggs at the 

 margin of these holes on the under side. 

 When the larvae hatch they make their 

 way to the roots, upon which they feed. 

 The really remarkable thing about this 

 larva is how it gets its air supply, as it 

 does not have gills, nor is it known to 

 visit the surface for a fresh supply of air, 

 and yet it has a normal air-breathing sys- 

 tem. On the dorsal surface, near the tail 

 end of the body, are two slender, curved, 

 spine-like processes. The air tubes of the 

 body arise from the base of these spines, 

 and spiracular-like openings are found at 

 their base. 



Two diflferent views have been advo- 

 cated to explain how it is possible for the 

 larva to secure air. There seems to be 

 no difference of opinion with regard to 

 the source of the air supply, from the air 

 cells in the root of the plant upon which 

 the larva feeds. One view is that these 

 air spaces in the plant are punctured by 

 the spines and thus the air is taken di- 

 rectly into the air tubes. The other view 

 is that the larva bites a hole into the air 

 space and then, by the aid of the spines, 

 holds the openings at the base of the spine 

 against the air space and thus the air is 

 taken up. 



The Back-swimmers are curious little 

 fellows which swim upside down in the 

 water, and by means of their sucking 

 mouth parts, prey upon other small ani- 

 mals. The lightness of their bodies and 

 the large amount of air which they carry 

 with them make it necessary when they 

 wish to remain below the surface to hold 



