1864.] 617 



wood and bark, so far as we can estimate it by the eye, is proportion- 

 ably ten times as great as with any gall produced by a Cxall-gnat or 

 Gall-fly or Saw-fly or Grall-moth or Plant-louse, yet never, so far as I 

 have noticed, kills the limb on which it grows. The reason is obvious. 

 In a true gall, made for example by a Gall-fly, besides the lesion of the 

 woody fibre and bark, there is, as I have shown, (P. E. S. P. II. pp. 

 472 — 6,) poison infused into the wound, the result of which is gene- 

 rally death, unless the poisoned limb is very large and vigorous. In a 

 pseudo-gall there is no such poison infused, and the damage done is 

 simply what would be done, if we were to take an auger and bore the 

 same quantity of wood and bark out of the limb. Whence we may 

 draw this Corollary, that the Black-knot is probably a true Gall ; and 

 as from its structure it is manifestly not the work of a Gall-fly or of a. 

 Plant-louse, or of a Gall-moth — for the Galls of Gall-flies always contain 

 hard, seed-like kernels and the Galls ot Plant-lice and of Gall-moths, 

 so far as my experience extends, are hollow — it follows that it must be 

 the work of a Dipterous fly or else of a Saw-fly. But if it was the work 

 of a Saw-fly, surely Dr. Fitch must have noticed its larva, so closely 

 as he examined the gall ; for the larva? of Saw-flies are pretty large 

 and may be recognized at a glance. Therefore it follows by the method 

 of exhaustion that it must be the work of a Dipterous fly; and as there 

 are but two Gall-making Dipterous families. Tri/prtidse and Cecido- 

 mijhlx, and the former is poor and the latter exceedingly rich in species, it 

 is most likely the work of some Cecidomyidous insect. 9^A. As already 

 stated (p. 578, note), I have found on the wild plum, galls strongly re- 

 sembling the Cecidomyidous gall >S'. ht-msicoides, and which I have 

 no doubt whatever are, like that gall, Cecidomyidous; and, unless my 

 memory deceives me, I have noticed on the leaves of the wild plum in 

 considerable quantities tubiliform galls strongly resembling the Ceci- 

 domyid()us hickory-gall Tublcola (). S. Now I believe that it is a gen- 

 eral law with gall-insects, that where one species of a particular genus 

 exists on a given genus of plants, many other species of the same genus 

 or of elo.seIy allied genera coexist with it. (See Proc. Ent. Sor. 

 Phd. II. 461-2.) But. with the two exceptions just referred to, there 

 is no Gall-fly or Saw-fly or Plant-louse or other gall-making insect 

 known at present, so far as I am aware, to form galls on the Plum- 

 tree. Hence if the Black-knot is the work of insects — which in spite 



