14 FIFTH REPORT OF THE ENTOMOLOGICAL COMMISSION. 



parent. A second generation of sexless individuals thus results, which is succeeded 

 by a third, fourth, fifth, and even a ninth generation, the process being only termi- 

 nated by the approach of cold weather, when a last brood of males and females ap- 

 pear. By this anomalous, asexual mode of reproduction, a single Aphis may become 

 the parent of millions of young. 



Certain plant-lice occur on the roots of plants, others on the stems or twigs; others 

 puncture leaves, causing them to roll or crumple, or to form galls. Ants are fond of 

 the sweet excretions from the " honey tubes," and often keep them captive in their 

 nests like herds of cattle. The maggots of Syrphus flies, lady-birds (Coccinella), and 

 the larvae of the lace-winged fly, besides small ichneumons, destroy great numbers of 

 them and keep them within due limits. 



To the plant-lice family belong the species of Adelges and Cliermes which produce 

 cone-like swellings on the new-grown twigs of spruce; also of Pemphigus, which pro- 

 duce gall-like swellings on poplars, etc. 



Bark-lice. — In the species of Goccidce, the males alone are winged, 

 having but a single pair, while the females are wingless, scale-like and 

 do great damage by puncturing the bark of trees. 



Dipterous or two winged gallflies. — Maples, wild plums, poplars, and 

 other trees have numerous leaf-galls of varied form made by little gnat- 

 like flies belonging to the dipterous family Gecidomyidce. These flies 

 are minute, most of them smaller than a mosquito. The females lay 

 their eggs in the stems, leaves, and buds of various plants and trees, 

 thus producing galls, a common example being the willow dipterous 

 gall-fly ( Gecidomyia strohiloides). There are thirteen other species found 

 by Mr. Walsh to raise galls on eight different kinds of willow, the dif- 

 ferent kinds of galls being readily distinguished, while the flies them- 

 selves and their maggots are closely similar. The maggots of the 

 Cecidomyians are usually minute orange, pinkish, or yellowish worms 

 without feet, and with the body pointed at each end. 



Insectivorous or parasitic insects. — WhWa the undue increase of forest 

 insects is largely prevented by insectivorous birds, their numbers are 

 especially reduced by the attacks of parasitic or carnivorous insects. 

 Of these the most efficient are the ichneumon flies, which are wasp- 

 like insects forming a large group of the order Hymenoptera, belonging 

 to the families Ichneurnonidce, Proctotrupidce, and Ghalcididce. Of the 

 ichneumons there are probably from 4,000 to 5,000 species. Many of 

 the species of Proctotrupidce oviposit in the eggs of Lepidoptera and of 

 dragon flies, etc. The largest species belong to the first named family. 

 They are recognized by their long, slender body and long, external 

 ovipositor. The larva is like the maggot of a bee or wasp, being foot- 

 less, soft, and white, and with a smaller head. 



'^When about to enter the pupa state the larva spins a cocoon, 

 consisting in the larger species of an inner dense case and a looser, 

 thinner outer covering, and escapes as a fly through the skin of the 

 caterpillar. The cocoons of the smaller genera, such as Cr^^ptus and 

 Microgaster, may be found packed closely in considerable numbers, 

 side by side, or sometimes phiced upright within the body of cater- 

 pillars."* 



* Packard's "Guide to the Study of Insects," p. 193. 



