REPORT ON FORESTS. 209 



twigs of older trees. It is fully i }i inches in diameter as a rule, 

 and sometimes considerably more. Through the outer envelope 

 comes, in due time, a series of pointed processes which are, each 

 of them, larval cells : in other words, instead of the gall being 

 produced by and containing only a single larva, some 50 to 100 

 have combined to form the larger swelling, in which each larva 

 has a separate cell. 



In some cases the very tips of the shoots become swollen and 

 distorted, so that the gall is really a malformation of the shoot 

 itself ; or there may be a cluster of soft ovoid growths so close 

 together as to press themselves out of shape and produce a series 

 of irregular, more or less transversely flattened blisters. 



More rarely galls occur on the roots, and these are usually 

 irregular, lumpy swellings. 



But not the Hymenoptera alone produce galls : quite a num- 

 ber of Diptera, or flies, belonging to the family CecidomyidcB 

 produce abnormal growths on leaves or twigs. One species 

 attacks the base of the leaves of pines and causes an abnormal 

 onion-like swelling at that point ; quite a number are found 

 making galls on the hackberry, while the willow is especially 

 favored by them : all sorts of abnormal growths being produced, 

 from little swellings to small cabbage-heads. 



A few of the beetles are gall-makers — some on pine, like 

 Podapion gallicola^ others on deciduous trees — and these galls 

 are usually mere swellings on the twigs or branches, in which 

 the larvse feed. 



Phylloxera galls are common on hickories, and it is not an 

 uncommon thing to see a young tree with the leaves covered 

 with the large blister-like galls which are open inferiorly. If 

 one of these galls be cut open, the inner surface will be found 

 so crowded with the insects that they can scarcely find room to 

 insert their beaks to obtain food. 



Plant-lice, belonging to the same order as the Phylloxera^ also 

 produce more or less obvious gall-like growths, and the elm is 

 a favorite tree for their attack. Sometimes there is a mere 

 curling and distortion of the" leaf, like that caused by Schiz- 

 ■oneura ainericana^ but often a real swelling, like the cocks-comb 

 gall made by Colopha ulmicola^ is produced. 



It seems, thus, that most of the orders of insects outside of the 

 Neuropterous series contain gall-makers, and all of them are, of 

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