HOUSE FLIES AND BLUEBOTTLES 213 



annoyance in the premises we occupy, damaging our 

 food or other property, attacking our persons, or worry- 

 ing and harassing our nervous susceptibilities. Some 

 interesting questions arise in connection with this 

 undesirable intimacy of relation, but many more ob- 

 servations will be needed before any very satisfactory 

 answers can be given to them. It is not easy to under- 

 stand, for instance, why these particular species of flies, 

 rather than any others, have elected to attach themselves 

 to man, and to follow his fortunes, as some of them have 

 done, all over the world. It is not that they are so 

 different from other flies that one would necessarily 

 expect them to behave in an exceptional way ; neither 

 in structure nor even in habits, except in this one 

 particular, is there anything which will broadly dis- 

 tinguish them from allied species which do not trouble 

 us. There is absolutely nothing that would enable a 

 person ignorant of the species to separate, in a given 

 assortment of flies, those that are household pests from 

 those that are not. We get one from one group, another 

 from another, and so on, but they do not form a compact 

 and isolated company. Their association with man, it is 

 true, is not so complete as that of several other insects, 

 such as the cockroach, the clothes-moth, and the bed- 

 bug, which spend their whole lives under the shelter of 

 our houses, and propagate themselves generation after 

 generation, without ever troubling themselves about the 

 outside world. As already mentioned, it is only in the 

 last stage of their life that, as a rule, we are annoyed 

 by these flies; but perhaps this limitation may be re- 

 garded as making the association all the more remark- 

 able. That as the perfect stage is reached in each 

 succeeding generation, the instinct to betake itself to 

 the abodes of men should regularly recur to an insect 



