October, '16] DAVIDSON; SYRPHID^ IN CALIFORNIA 455 



to find this latter type of aphidids attacked before they have secured 

 a good start, a phenomenon which suggests a balance of nature which 

 strives to provide enough food for future generations of syrphids by 

 guarding against the wholesale destruction of the young stem-mothers. 

 If the adult flies that issue early in spring confined their attention to 

 the young stem-mothers of the orchard aphidids, the latter would be 

 either wiped out or so restricted that there would not be food for the 

 future generations of flies. 



Throughout summer and fall the larvie of Catahomba pyrastri attack 

 ail varieties of injurious aphidids that they can reach. The species 

 must be credited with enormous destruction among plant-lice. All 

 things considered it is one of the most beneficial insects in California, 

 vieing with such well-known species as the Vedalia {Novius cardinalis), 

 the Chalcidine scale parasites, and Hippodamia convergens. 



In California there are about 14 species of Syrphus, 9 of which are 

 either quite uncommon or confined to mountain districts. * However 

 S. rihesli L., S. torvus Osten Sacken, S. americanus Wied., S. opinator 

 Will., and S. arcuatus Fallen attack injurious plant-lice in appreciable 

 numbers. Very beneficial are the two most abundant of them, S. 

 americanus and S. opinator whose larvse are general feeders and may be 

 taken frequently among colonies on cultivated plants. The larvae 

 of S. arcuatus occur in early spring on coniferous plants feeding upon 

 aphidids occurring thereon and later they attack principally such semi- 

 protected species as Aphis malifolice Fitch on apple. Like torvus and 

 ribesii, arcuatus is common to Europe and North America. All the 

 members of the genus in California have yellow or reddish-yellow 

 transverse bands on the abdomen. In arcuatus the three principal 

 bands are interrupted, while in the others above mentioned only the 

 first band is interrupted. The larvae are brown, yellow or purple and 

 the puparia brown. The larvse are not so large or voracious as those 

 of Catabomha pyrastri. Closely aUied to Syrphus is a common species, 

 Eupeodes volucris Osten Sacken. This fly is marked like the members 

 of the genus Syrphus but is somewhat smaller, albeit large examples 

 of the female are almost indistinguishable from the female of Syrphus 

 arcuatus. The larvje have been bred from injurious and other plant- 

 lice. 



The genus Sphcerophoria is represented in California by three species, 

 Sph. melanosa Will., Sph. sulphuripes Thomson and Sph. micrura 

 Osten Sacken, of which the two first at least are aphidophagous in the 

 larval stage. These are common flies, narrow-bodied and small, and 

 are peculiar in having extra segments to the abdomen. They are 

 blackish with bright yellow cross-bands. The larvip and puparia are 

 green and the pupse commonly occur on the plant on which the larva) 



