116 NATURAL ARRANGEMENT OF INSECTS. 



deed, the occurrence of apterous genera is a circum- 

 stance far from uncommon in all large groups of insects ; 

 and it perhaps merits consideration whether, naturally, 

 Latreille's solitary Heterogyna do not disperse themselves 

 throughout the aculeated Fossores, instead of being col- 

 lected together ; which is, possibly, merely a systematic 

 convenience, although it is in some measure confirmed 

 by the structure of the prothorax of the males. The 

 numerous typical genus Mutilla offers a repeated in- 

 stance of what we had occasion to observe in the genus 

 Halictus ; the divergence, namely, of extensive genera 

 from their types. We are acquainted with nearly 200 

 species of this genus ; and it is, perhaps, the most nu- 

 merous, excepting only Ichneumon, throughout the Hy- 

 menoptera. The occurrence of pectinated antenna is 

 extremely rare amongst the fossorial aculeates ; we have 

 seen it only in Psammotherma, belonging to this group, 

 but it is said to be found also in one of the PonipHidce, 

 among the typical Spheces. Apferogyna is remarkable 

 for the constriction of the basal segments of the ab- 

 domen in both sexes, and also for the evanescence of 

 the apical nervures of the wings. Thyrmus and its 

 numerous cognate genera enter here, for its females 

 are all apterous. Many species, and the legitimate 

 partners of several of them, are now known to us: the 

 latter, however, could only be ascertained in consequence 

 of the difterences of their colour, sculpture, and form 

 from the males, from the direct observation of friends 

 in New HoUand, where, with the exception of two or 

 three genera, the whole of the family which they ne- 

 cessarily constitute are found. It is into this family 

 doubtlessly that the Ells of Fabricius {Mysine partly of 

 Latr.) enters; for they cannot longer be associated with 

 My sine (Plesia Jurine), as we know the males of the 

 latter. The females of the Thynnidce, especially those 

 of the more typical genera, present many remarkable 

 structural characters, in the form of their legs, and of 

 the terminal segment of the abdomen ; and in seme 

 tliere are singular longitudinal and transverse deep sul- 



