OYSTEK-SHELL BAEK-L0TJ8E. 35 



The following extract from this article presents some interest- 

 ing observations not otherwise stated in the body of this report. 



The causes which have been instrumental in producing the de- 

 struction of the Oyster-shell Bark-louse, aud which are still opera- 

 ting to its completion, are matters of much interest. The agencies 

 to which it has been usually attributed are the four following : In- 

 sectivorous birds ; predaceous insects, especially the Coccinelloi^ 

 or Lady-bugs, and their larvae ; the larvae of the parasitic Chalcis- 

 flies, and the Mites, or Acari. 



It has been generally supposed that the smaller insectivorous 

 birds, such as the wrens and warblers, devour many of the eggs of 

 the Bark-louse, but these eggs are so minute and so completely 

 concealed under the bark-like scales, that even the sharp eyes of 

 a bird could scarcely detect them, unless it were endowed with a 

 special instinct for the purpose, and I know of no record of any 

 actual observations which confirm this supposition. I am there- 

 fore inclined to the opinion that birds have done little or nothing 

 in the way of exterminating the Bark-louse. 



The CoccinelloB devour a very small proportion of these insects, 

 whilst they are in their incipient and active state ; but this lasts 

 only three or four days, and therefore but very few of them can be 

 thus destroyed. These predaceous insects, and especially their 

 larvae, also destroy a few of the bark-lice, in their subsequent sta- 

 ges, by gnawing ragged holes through the scales, and thus getting 

 access to the insect beneath. Mr. Walsh conjectured that these 

 rough holes were made by Acari, but I have repeatedly seen the 

 larva of the Two-spotted Coccinella in the act of gnawing just such 

 holes in the scales of the Bark-louse of the pine tree, and devour- 

 ing its contents, and it is therefore probable that they are the au- 

 thors of the similar holes on the apple tree. But the small num- 

 ber of scales eaten into shows that but few bark-lice are destroyed 

 in this way. 



The destructive work of the Acari is supposed to be indicated by 

 the brownish, discolored remnants of the eggs from which the 

 contents seem to have been extracted, easily distinguished from 

 the pure white shells from which the insects have been hatched. 

 Both Mr. Walsh and Dr. Shimer, who were the first to notice 

 these mites, attribute much efficacy to their depredations, but that 

 they are the sole authors of this work is rendered somewhat doubt- 



