10 FIFTH REPORT OF THE ENTOMOLOGICAL COMMISSION. 



with reddish browu patches, so that these irregular spots, wheu the 

 cateri)ilhir is at rest, are closely similar to the dead and sere blotches 

 so frequent on oak leaves. The same may be said of other kinds feed- 

 ing on the leaves of other forest trees. 



While the bodies of those ]!«loctuid caterpillars which feed on herba- 

 ceous plants are smooth, those of the tree-inhabiting Catocala, Homop- 

 tera, and Pheocyma are mottled with brown and ash like the bark of the 

 tree, and provided with dorsal humps and warts assimilated in form 

 and color to the knots and leaf scales on the twigs and smaller branches. 



There is thus a close harmony in color, style of markings, shape, and 

 size of the humps and other excrescences of tree-inhabiting caterpil- 

 lars, and it is due to this cause that they are protected from the attacks 

 of their enemies. Mr. Poulton has recently called attention to the fact 

 that caterpillars are extremely liable to die from slight injuries, owing to 

 their soft bodies and thin skins. They can not defend themselves when 

 once discovered. The means of protection are of passive kinds, i. e., 

 such as render the delicately organized animal practically invisible on 

 the part of its enemies, and these means vary with each kind of cater- 

 pillar. In this way different kinds of larvfe can live on different parts 

 of the leaf, the upper or under side, or the edge ; on different colored 

 twigs, on those of different sizes, with different kinds of leaf scars, 

 scales, or projections ; and thus the tree is divided, so to speak, into so 

 many provinces or sections, within whose limits a particular kind of 

 worm may live with impunity, but beyond which it goes at the peril of 

 its life. 



To the Mymenoptera belong the gall-flies and saw-flies, besides bees 

 and ants, and ichneumons. 



GaU-Jlies. — These little creatures produce tumors or galls both in the 

 trunk, branches, but more usually the smaller twigs and leaves of the 

 oak, and rarely other trees. They belong to the family Cynipidw, and 

 are described as follows in the writer's "Guide to the Study of Insects:" 



The gall-flies are closely allied to the parasitic Chalcids, but in their habits are 

 plaut-parasites, as they live in a gall or tumor formed by the abnormal growth of the 

 vegetable cells, due to the irritation first excited when the egg is laid in the bark or 

 substance of the leaf, as the case may be. The generation of the summer broods is 

 also anomalous, but the parthenogenesis that occurs in these forms, by which im- 

 mense numbers of females are produced, is necessary for the work they perform in 

 the economy of nature. When we see a single oak hung with countless galls, the 

 work of a single species, and learn how numerous are its natural enemies, it becomes 

 evident that the demand for a great numerical increase must be met by extraordinary 

 means, like the generation of the summer broods of the plant-lice. 



The gall-flies are readily recognized by their resemblance to certain Chalcids, but 

 the abdomen is much compressed and usually very short, while the second, or the 

 second and third segments, are greatly developed, the remaining ones being imbri- 

 cated, or covered one by the other, leaving the lined edges exposed. Concealed 

 within these is the long, partially coiled, very slender ovipositor, which arises near 

 the base of the abdomen. [See Plate xv, ovipositor of the gall-fly.] Among other 

 distinguishing characters, are the straight (not being elbowed) thirteen to sixteen 

 jointed antennae, the labial palpi being from two to four jointed and the maxillary 



