10 FIFTH REPORT OF THE ENTOMOLOGICAL COMMISSION. 
with reddish brown patches, so that these irregular spots, when the 
caterpillar 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 lorest trees. 
While the bodies of those Moctuid caterpillars which feed on herba- 
ceous plants are smooth, those of the tree-inhabiting Catocala, Homop- 
fora, and Pheooyma 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 thiu 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 larvae 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 Hymenoptera belong the gall-flies and saw-flies, besides bees 
and ants, and ichneumons. 
Gallflies. — 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 Cynipidce, and 
are described as follows in the writer's u Guide to the Study of Insects:" 
The gall-flies are closely allied to the parasitic Chalcids, but in their habits are 
plant-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 antennas, the labial palpi being from two to four jointed and the maxillary 
