INTRODUCTION 



ROBBER FLIES OF THE WORLD 



sini from the Laphriini and also the Andrenosomini 

 with the exception, that in some species of Proagonistes 

 Loew there is a medial seam on the arch. The Andre- 

 nosomini, with eleven genera, are regarded as an off- 

 shoot from the Laphriini, as are likewise the Ctenotini. 



Except for the Ommatini, the Asilinae have not 

 been subdivided. The Asilinae segregate compara- 

 tively well into those genera in which the anterior 

 branch of the third vein lies above the wing apex and 

 those where it falls below. The first group is filled 

 with such diverse types as Promachus Loew, Procta- 

 canthus M acquart, and Polysarca Schiner. The second 

 group is even more variable, so I have decided to leave 

 them all in one group. The Asilinae, if they include 

 the more ancient known fossils, are also the most suc- 

 cessful present day forms, dominating as they do fields 

 and grasslands. Powerful, active and aggressive, these 

 larger forms are matched in ferocity only by the spe- 

 cies of Microstylum Macquart in the Dasypogoninae. 

 The larger asilids do not hesitate to defend themselves 

 when captured by the collector and the unwary will 

 be quickly and viciously bitten by them. 



The palpus has been reduced from two segments to 

 one in at least five groups — the Asilinae, Leptogas- 

 trinae, the Megapodinae, in Enigmomorpheus Her- 

 mann, and in the Ctenotini. The marginal cell has 

 become closed five times in this family — in the Asilinae, 

 some Laphystini, all Laphriinae, most Megapodinae, 

 and in all Thereutrini. 



It seems apparent that the Asilidae are better de- 

 veloped today, both in point of genera and species 

 than in the early or middle Tertiary. Compared with 

 the family Syrphidae, we find that today the Asilidae 

 has about the same number of species, though a great 

 many more genera and curiously more subgenera. But 

 there are almost certainly many more species of asilids 

 undescribed than there are of syrphids. Known fos- 

 sil species of Asilidae total 39, whereas the syrphid 

 fossil species number 75, and of this number, 30 are 

 from the Baltic amber, against only 4 asilids from 

 Baltic amber. However, the habits of asilids are per- 

 haps not such that amber would make a likely trap for 

 these flies. If the amber fossil syrphids are subtracted 

 from the number of known fossils in this group, there 

 still remain 45 syrphids from rock or shale formations 

 compared with 35 asilids from similar formations. 



In each of the larger subfamilies of the Asilidae there 

 appears to be at least one or more actively differentiat- 

 ing or evolving groups. For the Dasypogoninae this 

 would seem to be the Laphystini, the genus Cophura 

 Osten Sacken with its related genera and the genus 

 Microstylum Macquart with its related genera. For 

 the Laphriinae, the presently best developed group 

 would be the Atomosini; for the Asilinae, the Nerax, 

 new genus, group of species and genera and perhaps 

 Promachus Loew. 



Several trends in the family appear to have resulted 

 in well marked phylogeronts. One of these is giant- 

 ism, which appears several times in the family, in 



Phellus Walker and in some species of Microstylum 

 Macquart. Clumsiness, as well as giantism, is appar- 

 ent in some species of Bombomima Enderlein, and 

 they have become about as robust laterally as they can 

 well be, and some of these lay very large eggs, with 

 consequent reduction in number, though admittedly 

 the hazards for initially large larva may be less, as 

 far as our present knowledge goes. If, as some believe, 

 these larva are predatory, this too may well be a lim- 

 iting factor in these species. I believe we must regard 

 Phellus Walker species, Obelophorus Schiner, Dasy- 

 lechia Williston as phylogeronts; perhaps too, all or 

 most of the Megapodinae are such. Dwarfism appears 

 in the Stichopogonini, as well as in some minute and 

 fragile species of the Leptogastrinae and Atomosini. 

 There are some parallel developments as in species of 

 both Acronyches Hermann and Systellogaster Her- 

 mann, which resemble flies of other families. One 

 species of Acronyches Hermann has the milky wing 

 apex seen among the Syrphidae, in similarly petiolate 

 species of Microdontinae and Cerioidinae, and in 

 wasps. Most remarkable is the genus Eurhabdus Al- 

 drich with the threadlike abdomen, long-stalked hal- 

 teres and wing reduced in the basal half to a mere 

 stalk. 



Morphology 



The robber flies are characteristically large, active, 

 aggressive flies, which tend to have bristly vestiture, 

 a short thorax and a stout, yet tapered abdomen. The 

 abdomen, in contrast to therevids, tabanids, nemestri- 

 nids and bombyliids, is always reduced in girth at 

 the point of attachment to the thorax. This latter 

 condition is probably connected with the hovering 

 habit present in all four of these other families, and 

 possibly in several of them is associated with air sacs. 

 Asilids are able to hover but do so uncommonly. The 

 robber flies are very efficient predators. They either 

 pounce on their prey or capture it in flight with great 

 agility. Surely the unusual development of the legs, 

 so often stout and swollen and beset with strong bris- 

 tles, is connected with this predatory habit. The eyes 

 are prominent and well developed, without exception 

 widely dichoptic in both sexes, and the front of. equal 

 width in both. In a few Leptogastrinae the eyes may 

 virtually meet beneath the antenna. This is the gen- 

 eral picture of the family, but with so many genera 

 and species there are numerous exceptions. Some asi- 

 lids are weak flies with few bristles and delicate in 

 body. 



The head in most asilids is of medium length. In 

 a few instances, especially in some Damalini, it is 

 shortened, the eye being high, short, and flattened. 

 The postocular occiput or that part which shows be- 

 hind the eye profile is generally prominent and some- 

 times especially well developed. Occasionally the 

 upper occiput is obliterated by the posterior develop- 

 ment of the eye, but the great uniform expansion of 

 the eye with concave, posterior occiput, such as in 

 tabanids, some bombyliids, and nemestrinids is never 



