1NTK0DUCTI0N 



ROBBER FLIES OF THE WORLD 



15 



Text-Figure 6. — The immature stages of Asilidae (after Melin, 1923) : A, Larva of Machimus atricapillus Fallen. B, Larva of Dysmachus 

 picipes Meigen (as jorcipula Zeller). c, Larva of Choerades gilva Linne. D, Pupa of Machimus atricapillus Fallen. 



that about 500 or more species occur in California ; this 

 may be attributed to the very diverse habitats and cli- 

 mates, of the State and to its length. 



Robber flies begin to move several hours after sun 

 up and are occasionally active until sunset. Gener- 

 ally speaking, they are most active from 10 in the morn- 

 ing until 2 in the afternoon and are strongly affected 

 by hot, bright sunshine. A few species have short lives 

 of but a week or ten days ; aggregates of other species 

 may live for six months. In the southern United 

 States Nerax interruptus Macquart, a ubiquitous 

 species, appears early in June and lasts until Novem- 

 ber or later, depending on the appearance and severity 

 of frosts. This species is called by the southern ne- 

 groes, who are country-living, by the peculiar folk 

 name "louanners." 



It is surprising how limited the population may be 

 in some species of Asilidae. A population of 50 or 

 more individuals of a species in an area of limited 

 extent may be found, and it may be a very consider- 

 able, even remote distance to other populations. I re- 



call finding a population of Asihis rubietmdus Brom- 

 ley in a hilltop plot of field grass beside a highway. 

 There were scattered patches of woods and fields total- 

 ing hundreds of acres, but these flies were centered in 

 a small area of not more than one-half acre and in a 

 3-day period I netted 152 individuals. It was esti- 

 mated that 20 more remained. A search the same 

 days in a number of similar areas, including those 

 where formerly the species was located, failed to re- 

 veal others. Two days of collecting once yielded over 

 100 individuals of the very large Proctacanthus mil- 

 berti Wiedemann from two acres bordering a swamp, 

 and other individuals remained. Similar isolation of 

 populations was noted by me in Australia. On the 

 other hand, there are species and genera and tribes 

 which are particularly abundant and can be found at 

 most any time and place within the geographic range 

 and proper habitat. Many asilids frequent the bark 

 of trees or logs, drift, wood, mud flats or the twig tips 

 of small trees and shrubs. Some are restricted to pine 

 forests, a not surprising fact in view of the extensive, 



