188 



KNOWLEDGE. 



[August 1, 1890. 



however, in the slightest degree incommoded. Another 

 external parasite sometimes, but not so commonly, found, 

 is an animal belonging to a group closely allied to the 

 mites, viz. the book-scorpions or C'helifers. It is a little 

 reddish creature wdth a pair of great pincers in front like 

 a scorpion, but differing in that the body does not taper 

 away into a tail, but ends abruptly. Amongst tlie internal 

 parasites are ^•arious kinds of small hynienopternus 

 insects, aUied to the ichneumon flies ; and an instance is 

 recorded of an exceedingly line and hair-like uematoid 

 worm, of the enormous length of three inches, liaving been 

 taken from the abdomen of a house-fly. 11. Fourment, 

 who records the fact, states that notwithstanding that the 

 parasite had caused a considerable enlargement of the 

 body of its host, the latter did not seem in any way in- 

 convenienced in its flight. 



We h;ive now enumerated eight species of Miiacidtr 

 whicii ai'e more or less intimately associated with man, 

 and which, either by reason of some peculiarity in their 

 economy, or simply in consequence of their numerical 

 abundance, often become a source of trouble and annoy- 

 auce in the premises we occupy, damaging our food or 

 other property, attacking our persons, or worrying and 

 harassing our nervous susceptibilities. Some interesting 

 questions arise in connection with this undesirable inti- 

 macy of relation, but many more observations will be 

 needed before any very satisfactin-y answers can be given 

 to them. It is not easy to undeistand. for instance, why 

 these particular species of tlies, 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 distinguish 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 cockroacla, the clothes moth, and the bed-bug, 

 which spend their whole lives ixnder the shelter of our 

 houses, and propagate themselves generation after genera- 

 tion without ever troubling themselves about the out- 

 side 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 regarded as making the association all the 

 more remarkable. 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 

 born and bred in the open air, is, it would seem, more 

 remarkable than that the association should be a con- 

 tinuous and permanent one. As the nature of the food on 

 which they are reared necessitates, as a rule, that they 

 should pass through their earlier stages exposed, it is 

 rather curious that the perfect insects should not confine 

 themselves to similar localities, but should also enter our 

 dwelhngs, and often in such surjirising numbers. 



Nor is it, agam, that they are so much more abundant 

 than all other species, and that, therefore, mere excess 

 of numbers causes them to be the species represented 

 indoors ; that, in other words, we simply get the overflow 

 from outside. Of course they are abmidant — this is im- 

 plied in their being pests — but there are other species 

 equally so, of which it is the rarest occurrence to find a 



specimen in the house. Take, for example, the case of 

 Sdn-oiiliaija nirxarui, the flesh fly, which has been several 

 times referred to already. This is an insect of most 

 extraordinary fecundity ; it is said that as many as 

 20,000 eggs have been found in the ovaries of a single 

 female, and, m consequence, it is an extremely connnon 

 fly ; but though its habits are similar to those of the blue- 

 bottle, and it swarms round human dwellings, it is very 

 seldom seen indoors. The facts of its distribution seem 

 to show that it is far less dependent on man, and far more 

 inclined to ignore his movements, than our household 

 pests. It is an extraordinarily hardy insect, and shows 

 wonderful powers of adaptation to circumstances. Even 

 in the matter of food, which is often such a critical point 

 with a larval insect, it can stand some degree of variation, 

 feeding not merely on meat, either fresh or putrid, and 

 wounds and ulcers on men and other animals, but even 

 on decaying vegetable matters and dung as well. Even 

 if half- starved, it will still undergo its metamorphoses, 

 though, of course, the perfect insects will be dwarfed. 

 Like several others, it can even withstand the action of 

 the digestive fluids of the stomach and intestine of living 

 vertebrate animals. Bernard introduced it artificially 

 into the stomach of a dog, but it 2>assed along the intes- 

 tine and was voided in the usual way alive ; Portchinsld's 

 similar experiment with a frog had the same result. In 

 the case of a little song-bird, however, the larva was dead 

 when voided, but stdl undigested. That so common and 



(A) Portion ok Left Ovart of Bujeuottle. comaimxc^ ahoi t 

 80 Eggs. 



(B) Side View of Porjion of Right Ovary, showing Distri- 

 BexioN OF Ti!Acni;.E, ok Aik-Tihks (I), to the Organ. 



SO hardy a European fly should be one of the most likely 

 to follow man's lead and migrate with him to other parts 

 of the world would be only what was to have been ex- 

 pected, and yet, though the five flies mentioned at the 

 commencement of these papers, together with Ci/rtdiu-Kra 

 stoliulfois, are as common in the United States as in 

 Europe, though not indigenous there, S. airnnria has, 

 according to Osten-Sacken, not yet been introduced into 

 America, so that four centuries of European communica- 

 tion with the new world have not sufficed to import this 

 abundant but independent species. 



The curljus observations of Portcliinski have an 

 important bearing on the subject, though perhaps they 

 will hardly justify the conclusions he has drawn from 

 them. He finds that carrion -feeding flies are, as a group, 

 enormously prolific, while dung-feeding species are much 

 less so; for example, CiiUijilioni, a carrion-feeder, lays 

 from 300 to 600 eggs, while Mmca ihmestica, a dung- 

 feeder, lays only about 120. (Fig. 17 shows the method 

 of distribution of the eggs m the ovaries ; they lie side 

 by side in a compact mass.) These differences, he argues. 



