ISiVi.] 295 



b}' side, or dift'er only by very slight cluiracters, which would not be i;eii- 

 erally considered sufficient to separate two insects specifically. A case of 

 this kind is noticed below under Larhnus Can/se — a* gigantic aphidian 

 hitherto found only on the Hickory, but which I have found on the Hick- 

 ory, the Bass-wood, and the Oak. Perhaps, however, a more careful study 

 of these species in all their three states, might disclose distinctions, which, 

 if constant, might be of specific value. If, on the other hand, experiment 

 should prove, that a colony of one supposed species could be transferred 

 without injury to their health and procreative powers to a plant of another 

 family inhabited by another supposed species which closely resembled it, 

 the proof of their specific identity would be nearly complete. The whole 

 subject is obscure and requires further investigation. 



If we recur to the analogy of other families of Insects which have near- 

 ly the same habits as Aphidsc — for example Tiivjidm among the Hcterop- 

 tera — the difficulty is not lessened. As a general rule, so far as my own ob- 

 servation extends, each species of Tinjis is confined to some particular plant. 

 For instance, T. ciUata Say occurs only on the sycamore or buttuuwood 

 (platanus occideutalis), where I have noticed it in profusion on the uuder 

 side of the leaves along with its larva both in North and South Illinois. 

 But on the other hand, T. juglandis Fitch, which that author states to 

 breed on the butternut and to be "sometimes met with on birch, on wil- 

 lows, and other trees," is undistinguishable, so far as the brief description 

 of the imago goes, from a species which I found in profusion in South 

 Illinois on what I took to be an ash. Again, a third species, which so far 

 as I know is undescribed, occurs on the bass, the wild cherry, and the 

 false indigo (amorpha fruticosa), or at all events the imagos found in great 

 abundance on these three plants belonging to three distinct families, are 

 undistinguishable when placed side by side. Perhaps part of the difficul- 

 ty may arise from authors supposing that, because they found a species on 

 a particular plant unaccompanied by its larva, it must necessarily have 

 bred on that plant. 



But even if a species of Aphia found in company with its larva on one 

 plant differs obviously from another Apliis found in company with its larva 

 on another plant belonging to a different botanical family, it does not nec- 

 essarily follow, according to the general views of entomologists, that the 

 two are specifically distinct. There is a remarkable example in Lejiidop- 

 tera of a very considerable variation, correlated with variation in the food- 

 plant, in an insect feeding on plants of distinct botanical families, not be- 



