46 BRITISH FLIES 



sexual differences in the antennse are very pronounced in most of the 

 Nemoceka Vera of Osten Sacken, but disappear in his Nemocera 

 Anomala and in all the subsequent Diptera. 



The clothing is usually absent in the Nemocera, though coarse pu- 

 bescence of a peculiar nature occurs in many Bihionidce, Psychodidce, Culi- 

 cidce, and a few Mycetophilidce, and bristles may occur on the thorax and 

 lees of a different chsetotactic nature from those in the Brachycera ; but 

 it is not until the Therevidca are reached that regular strong bristles occur 

 in the Brachycera, but thence onwards through the Asilidce powerful 

 bristles occur on the thorax and legs, until they l;)ecome the rule in the 

 MiCROPiiOXA and subsequent families right away through the Athericera, 

 with occasionally lapses such as in Pipuncididce and Syrphidai In the 

 families after the Asilidce, the cephalic bristles begin to develop and soon 

 become an important feature. The first indications of bristles on the legs 

 show as apical tibial spurs, and then spicules occur in rows on the tibice 

 {Leptido:, Bomhjlida', etc.), and these spicules steadily develop until they 

 become conspicuous {Therevidm) and ultimately develop into strong 

 spines {Asilidce) ; a serration beneath the hind femora occasionally occurs 

 even in the Eremocii^ta {Xylomyia), but does not seem to bear much 

 affinity to the stick-like spicules which occur there in the Bomhylida;, nor 

 to the stouter bristles which are frequent in the Asilidce ; the serration 

 in the otherwise almost eremochaetous Mydaidce may be of a similar 

 nature to that which occurs in Xylomyia. The strong bristles on the legs 

 reach their highest development in the pedestrian Asilidce, and Osten 

 Sacken has pointed out that this character becomes weak in aerial families, 

 which accounts for its absence in Pipwicididce and Syiphidce. The soft 

 eremochsetous pubescence of the Sir atiomy idee becomes coarser in the 

 Tabanidce and assumes a slightly bristly nature in the Leptidca, after 

 which the first indications of macrochaetse occur in the densely furry 

 Bomhylince, but these macrochsetse are hidden away in the pubescence and 

 are sometimes more strongly developed in the females ; in the later sub- 

 families of the Bomhylidce (such as the Toxopliorince) the dense pubescence 

 becomes reduced and the macrochfetfe are correspondingly developed, 

 while the converse holds good in some Asilidce in which the normal strong 

 bristles on both thorax and legs are replaced by dense coarse pubescence 

 {Lcvphria, etc.). An affinity may exist in the facial and frontal bristles of 

 the Lonchop)teridce and Phoridce. The gradual growth of a minute pu- 

 bescence on the metapleura^ to the fan of bristly hairs or even strong 

 bristles which occur in most Asilidce (with a remarkable lapse in Stcno- 

 pogon) and Empidce is of interest, and in the Mnscidce (sciisu latissimo) 

 remarkable strong bristles of another type usually occur on the mesopleura^ 

 and metapleuroe. The postocular festoon is a row of strong bristles which 

 occurs in most Asilidce on the upper part of the back of the head and 

 rather overhanging the eyes, and the commencement of this may be found 

 in some Tabanidce and leptidoj, or may even be indicated by the erect 



