INTRODUCTION. 7 



need of food, and consequently their more predaceous habits. The 

 plastic differences distinguishing the male sex from the other may 

 be defined as follows : The eyes of the male are generally some- 

 what larger, the face and sometimes also the front narrower ; the 

 contiguity of the eyes in the male is not frequent ; still it takes 

 place in some Diaphorus above the antennae, and in some Chry- 

 sotus below them. The third joint of the antenna of the male is 

 usually somewhat longer, sometimes much longer than in the 

 female ; likewise the auteunal arista of the male is often much 

 prolonged, sometimes extended into a club or button at the tip or 

 enlarged in the shape of a lamel, whereas in the female the arista 

 is much shorter and quite plain. The feet of the females are, almost 

 without exception, plain ; those of the male often differ consider- 

 ably from them, and have various handsome ornaments, principally 

 on the fore and middle tibiae, and the fore and middle tarsi ; the 

 femora as well as the hind tibiae and tarsi very seldom show any- 

 thing but a plain structure. To these differences in the structure 

 of the feet must be added those derived from the hairs and bristles 

 which they bear; in the female these are usually more sparse, 

 shorter, and coarser ; in the male, closer, more delicate, and longer ; 

 sometimes also they assume in this sex some peculiar modified 

 structure. Even the ungues of the male are sometimes of a pecu- 

 liar irregular shape ; the pulvilli are in some cases (as in Diapho- 

 rus) larger in the male than in the female. The wings of the male 

 often differ from those of the female in the outline and the neura- 

 tion, those of the latter being in general more plain, and repro- 

 ducing in their neuration the characters common to the genus ; 

 whereas the wings of the male show in both respects more specific 

 peculiarities. These consist usually in characteristic sinuses of 

 the posterior margin and in a stronger sweep of flexure of the 

 longitudinal veins ; sometimes the anterior margin also shows a 

 peculiar curve in its outline, or a local thickening, or an elegant 

 fringe of hairs, all of which do not exist in the female. 



The hairiness of the eyes, as well as the hairs and bristles on 

 the other parts of the body, is frequently more dense, often con- 

 siderably longer in the male than in the female. The very minute 

 and dense tomentum with a silvery reflection, which adorns the 

 abdomen and the thorax in most species of Argyra, also forms 

 spots on the thorax of some other genera (as Pelastoneurus) and, 

 in the species of most genera, is perceptible at least on the lateral 



