326 AKKUAL RECORD OF SCIENCE AND INDUSTRY. 



oak of France, particularly of the department of Landes. 

 These three kinds of trees are closely allied, and so are the 

 insects found upon them, many insects common to one species 

 feeding indiscriminately upon the others; for example, al- 

 most all the beetles which live on the chestnut also prey on 

 the oak. An important practical question is answered by 

 Perris, whether the boring grubs of beetles attack healthy or 

 sickly trees. Under the influence of the authority of Ratze- 

 burg, and of facts imperfectly observed and appreciated by 

 him, the forestry schools, and indeed the entomologists, both 

 of Germany and France, liad admitted that wood-boring in- 

 sects attacked only healthy trees. But Perris insists that 

 such insects, which make their attacks in great numbers as 

 if acting by concert, and which consequently are very dan- 

 gerous, usually only infest sickly and enfeebled trees. " This 

 rule for it is one applies without exception, as I know, to 

 insects whose larvae pass their whole life or a part of it under 

 the bark, namely, to those which are the more numerous and 

 the more dangerous^ The circulation of the sap in the inner 

 layers of the bark of healthy trees, naturally very active, 

 become still more excited by the presence of the larvae, and 

 thus they become smothered, as I have seen from examples 

 thus killed." Certain beetles and other insects are an excep- 

 tion to this rule, such as Oompsidea^ etc., and there are spe- 

 cies of this genus in America which destroy elm and other 

 shade-trees while in health. Annales de la Soc. Linneemie 

 de Lyon^ 1876. 



THE EYE OF FLIES. 



Signor G. V. Ciaccio has published among the memoirs 

 of the Academy of Sciences of Bologna an account of the 

 anatomy of the eye of Diptera, describing the optic gan- 

 glion and optic nerve, the retina of w^hich he finds composed 

 of five coats, though still much more simple than in verte- 

 brates. He then describes the pigment, the external envelope 

 of the eye, and finally the tracheae distributed to it. Journal 

 de Zoologie. 



THE SEVENTEEN- YEAR CICADA. 



Professor C. V. Riley has shown that there are thirteen 

 as w^ell as seventeen year races of this Cicada, and has pre- 



