The f^outh Austratian Naturalist. 125 



No migrations of back-swiinmers have been recorded in 

 Australia, but migrations en masse must occur, for myriads of 

 adults sometimes appear in dams, etc, which a day or two 

 before were destitute of bug life. In 1847 the migration of 

 a tremendous host of an American species was observed hy 

 Sir George Simpson. The recorder of the occurrence says that 

 the bugs were suddenly incapacitated through encountering 

 a freezing stratum of air, and fell to the ground in countless 

 numbers, covering a space 25 to 30 miles in length and of a 

 width unable to be determined! 



The back-swimmer now dealt with is one of our common 

 est aquatic bugs, and in the breeding season sometimes occurs 

 in such very great numbers that hundreds may be captured 

 with a single sweep of the net. Although somewhat shy, it is 

 easily maintained ni aquaria ; it readily feeds upon mosquito 

 larvae and must therefore be considered of some economic 

 importance. A preliminary examination of the insect itself 

 will repay our interest. The first adaptation which may be 

 noted is that of the underside of the abdomen; as is usual with 

 the majority of the liack-swimmers, this has a median keel or 

 carina, on each side of which is a trough. Guard hairs close 

 over these troughs which thus form reservoirs for the storing 

 of air which is utilised during submergence. The legs are also 

 modified Tor an aciuatic habit ; the postei-ior pair are very long, 

 [lattened, and fringed with long hairs, and operate as paddles 

 or oars ; the anterior and middle legs are shorter, 'concave on 

 the inner sui'faces and with the edges set with long bristles 

 which serve for the retention of living prey. In the male the 

 anterior tibiae are further specialised, the base on the inner 

 side being expanded to form a spur on which is situated a 

 stridulatory comb; this comi) operates against chitinou;: 

 pi'ongs, which project one on each side of the beak, the friction 

 of the two mem])ers undei- water producing a shrill prolon^^n.^d 

 note. The fenmle lacks this stridulatory apparatus and difPej-s 

 from the male in several other minor characters. It is now 

 proved that the species under discussion inserts its eggs into 

 plants; if the ovipositor of the female is dissected out on a 

 mici'oscope slide, it will be seen that one of the pairs of for- 

 ceps or ''gonapophyses" of Avhich it is composed, consists of 

 two chitinous members, each with a saw-like edge of strong 

 teeth, and it is with these gouging gonapophyses that a recep- 

 tacle for the egg is prepared. An examination of the oviposi- 

 tor of other Australian members of the genus shows that it 

 is quite safe to assume that all of our species have the same 

 habit. 



