OF WASHINGTON. 413 



extent, also true of certain species of the family Anthomyiidae, of 

 which the larva is known to infest the egg-masses and nourish 

 from the eggs of locusts. 



I have already alluded to the chief characteristics of the 

 CEstridae, or Bot-flies, in treating of the animals affected by 

 parasites. Here, again, it is solely in the larva state that the inj.ury 

 is done. The egg is generally fastened by the female bot-fly to 

 the hair or such other parts of the animal attacked as may be 

 easily reached by the tongue, and is peculiarly constructed for 

 this purpose and also for the purpose of releasing the larva when 

 licked by the animal affected. In the stomach bots (Gastroph- 

 ilus), and even in the sub-cutaneous bots (Dermatobia and 

 Hypoderma), as recent researches have shown, the newly hatched 

 larva is structurally well fitted for wandering before it finally 

 becomes fixed for development according to the habit of the 

 species. Here, again, we have an illustration of the persistency 

 of habit, for on no other theory than that of derivation from 

 ancestral forms in which the larva was more free or non-parasitic 

 can we get an explanation of the circuitous method of reaching 

 its sub-cutaneous abode which is now known to be employed by 

 the young larva of Hypoderma. In CEstrus, as exemplified in 

 the Sheep Bot, we have a further modification of the same tendency 

 in that the ova are retained by the parent until they have either 

 hatched or are just ready to hatch, and the young larva is placed 

 in the nostrils of the sheep, where it can at once make its way to 

 the frontal sinuses. 



In the cases of Dipterous parasitism thus far cited it is the 

 larva only which is parasitic, the pupa being, with rare excep 

 tions, independent, and the imago also being free and non- 

 parasitic. 



We now come to the anomalous forest-flies and sheep-ticks 

 (Hippoboscidae). Here we find the normal Dipterous character 

 istics so modified that the insects have assumed many of the char 

 acteristics of the parasites of our first category ; for while some of 

 them are free in the imago state, others are confined throughout 

 their life-development to the host. They have a horny, flattened 

 body, recalling the lice in their general habits, and affect particu 

 larly birds and bats. The larva is hatched in the abdomen of 

 the female, which is capable of great distension. There it devel- 



