230 TRANSACTIONS OF THE ILLINOIS 



minutes, apparently sucking the sap of the tree. Thus its anatomical structure, and its 

 mode of life both demonstrate that it is a distinct species — an acarus that feeds on the 

 tree itself, and very probably is not confined to the apple tree. Besides this, I see other 

 acarians crawling about on the tree ; one a black fellow, that very much resembles those 

 sometimes found ou the bodies of Lady-birds, which doubtless is parasitic, and may feed 

 on apple bark lice ; but believing is not knowing. We must wait for observation before 

 deciding ; moreover it is not confined to apple trees. I have seen it on various forest 

 trees, especially prickly ash. I found a bark louse scale that cleaved in two between the 

 second and third segment or moult. An entire inside thin leaf separated from the outer, 

 heavier part of the scale, the insect being dead, and appears to have been killed very 

 soon after the third moult, before it had time to become cemented to the outer part of 

 the scale. This proves beyond doubt that the scale is made up of the successive skins of 

 the insect, cemented together. 



August 27th. — The egg laying season of the apple bark louse is fairly progressing. I 

 have not yet seen any acarians among the new eggs, but by looking under the last year's 

 scales, among the old despoiled eggs and shells, I find them numerous. They are of dif- 

 ferent sizes, the larger are about three or four times as long as wide, and evidently are 

 the mature Acarus Malus, those observed last year and this spring being younger. 

 These mature ones have a more prominent head, and the two long post-abdominal hairs 

 are shorter ; legs and feet corresponding with the published description ; color, white ; 

 motions equally slow with those described last year. 



September 13th. — The apple bark louse egg laying season has advanced to about the 

 close. 



I have closely observed the acarians during the month. The mature female is found 

 under the old scales always, so far as I have been able to determine. Here I also observe 

 the eggs, which are white and very small, about % the diameter of the egg of the apple 

 bark louse, but nearly or quite globular ; and here the young of different sizes are quite 

 abundant. They wander away in quest of food, and thus find their way to the new 

 scales, where they feast upon the new eggs, until they become much more round and 

 plump than any to be found under the old scales. 



From these observations I have developed the complete history of the acarian parasite, 

 discovered last year among the eggs of the apple bark louse, and described as Acarus 

 malus, and learned that the young appear abundantly in the early spring, and destroy 

 many young bark lice soon after they form their attachment to the bark ; that many 

 pass the summer under the old scales, and there they lay their eggs ; and in autumn the 

 young again sally forth and destroy many eggs. These acarians produce probably not 

 more than two broods in a season, and probably pass the winter in the egg state. 

 Whether those young that attack the eggs of the bark louse in the fall, live over winter 

 is uncertain, as in the spring I found only quite small young acarians. 



The fact that Mr. Walsh contemporaneously and independently discovered an acarian 

 enemy, of the apple bark louse, is collateral evidence of the wide spread existence of a 

 formidable enemy. Mr. Walsh says, that the young of his acarian had only six legs, 

 which if true, would indicate a diiferent species. But I am inclined to believe, that his 

 may be all eight legged, as were all of mine ; for with the best simple lens ever made, it 

 is difficult or impossible to determine the legs. My examinations were made with a 

 good microscope, one-twelfth inch objective, and therefore think that the slow traveling 

 acarians, which he observed are the same as my Acarus malus. 



