ENEMIES OF APHIDES. 399 



and, in some species of the nearly related dragon-flies, are pro- 

 vided with means of instantaneously reversing their action. 



In the ChrysopidcE^ as typified by C. perla^ the venation 

 of the wings is characterised by three or four longitudinal ner- 

 vures proceeding in determinate lines from the base of the 

 wing. In addition, some more or less zigzag veins connect the 

 numerous transverse veins in such a manner as to form an appro- 

 ximation to a longitudinal nervure ; but a slight examination of a 

 few species shows that this appearance is due merely to the coinci- 

 dence of the junctions of numerous short veins. 



The main character of the wing is given by the transverse 

 veins, which, dividing the longitudinal space into more or less 

 numerous cellules, give the net-like effect that has obtained for 

 these flies especially the name of " lace-wing." 



Both the anterior and posterior wings in the HemerobiincB are 

 studded with short, curved, and rather thick hairs along the lines 

 of the veins, becoming more marked in some species around the 

 edges and towards the base, and also about the point where more 

 or less indication of a stigma is sometimes found. No dark 

 mottling is found in the wings of Chrysopa^ but the green colouring 

 sometimes varies considerably, especially on the nervures. 



The wings of the HetnerobiidcB — one of which, carefully drawn 

 for comparison, is shown on PI. XIX., Fig. 6 — present less 

 appearance of transverse venation, and the main longitudinal ner- 

 vures are fewer, or, at any rate, are not so much in evidence. 

 The veins, which in Chrysopa are transverse, here become more or 

 less oblique or longitudinal, some of the short connecting veins 

 remaining to form the spaces between into cellules. These wings 

 are often freely motded with a considerable number of dark 

 patches, affecting the nervures as well as the membrane, and short 

 hairs are fringed pretty thickly along the lines of the veins, as well 

 as sparsely on the surfaces between. 



Many years ago, Mr. Bowerbank pointed out in the pages of 

 the Entomological Magazine (Vol. IV., p. 179) the beautiful phe- 

 nomenon of circulation exhibited in the wing venation of Chrysopa 

 perla. This interesting illustration of the vital processes of the 

 insect seems to have been but little noticed by microscopic observ- 

 ers, owing, no doubt, to the fly being rarely examined while living 



