SCIENCE- GOSSIP. 



99 



even more like bees than Erystalis, and live with 

 the wild or humble bees in their nests. 



Flies are of great use as scavengers, laying their 

 eggs on decaying animal matter. The larvae soon 

 hatch — no longer than in a day in the case of 

 the house fly — and find their food surrounding 

 them. The similar habit of the blow-fly is very 

 well known, spoiling meat in summer-time, the 

 larvae being the "gentle" so dear to anglers ; but the 

 harm done is probably more than balanced by the 

 good, as, m addition to the disappearance' of de- 

 caying matter, some very troublesome insects are 

 attacked in the same way. For instance, there is 

 nothing more harmful to plants than the aphis or 

 green fly. The larvae of the Syrphus-fly feed on them, 

 and it is a most curious sight to watch the ovi- 

 positing. The fly hovers over a rose or other bush 

 till it sees a leaf well covered with the green fly, 



Fig. 7. Diagram of small predaceous Fly. 



then settles quietly in the midst of them, lays an egg 

 or two, and then as quietly flies away, seemingly 

 conscious of having performed a virtuous action. 

 The flies of this genus get their popular name of 

 " hover " from this habit, and of " hawk " from 

 the male suddenly darting in pursuit of his partner. 



There are other flies that are well worth watch- 

 ing. Seeing the Scatophaga, the large hairy 

 orange-yellow fly, known to fishermen as the 

 " dung fly," seizing and killing a fly as large as 

 itself, is most dramatic. Note the little Sipliona 

 geniculata, a fly with a very long and thin proboscis, 

 which it pushes down the tubes of flowers such as 

 the privet and camomile to suck the honey, or 

 a blue or green bottle fly feeding on a disc of ivy 

 blossom and getting quite drunk with the effects. 

 I regret to say this dissipation is very popular 

 with several orders of insects. Watch the beauti- 

 ful Dolioliojms, a fly of vivid iridescent green, 

 with eyes shining as brightly as gold, skating on 

 shallow pools, like the water-spiders, and catch- 

 ing their food, which consists of minute insects, 

 worms, and even .slugs. Their legs are furnished 

 with very delicate hair, that holds the air and 

 prevents them from sinking. The mouth is also 

 furnished with two powerful teeth (fig. 9). 



However, it is not only flies that prey on flies ; 

 such innocent things as plants sometimes develop 

 a carnivorous taste. At Kew you can see the 

 tropical "Venus's fly-trap," and in England we 



have the marsh-plant, the " sun-dew " {Drosera, 

 rotvndifolia Linn.), which feed on flies. Early in 

 last century this habit was unknown, and observers, 

 seeing a number of a particular family on the 

 plant, concluded that they liked the secretion, and 

 named them Drosophilidae, or "Drosera lovers." 



Though flies do an immense amount of good in 

 the world, some species must be counted among 

 the enemies of mankind. There are few more cer- 

 tain sources to convey infection of disease from one 

 person to another than by several of the commoner 

 species. These infest the sick-room, and then stray 

 to other places, carrying the microbes of disease 

 on their feet or feeding organs. We can readily 

 imagine that some of these bacteria are left on 

 human food, such as meat or butter, which on 

 being eaten pass the infection to uninfected 

 persons. We have all heard also how certain 

 kinds of mosquitoes pass the microbes of malaria to 

 human beings, and the same may be said with 

 regard to yellow fever in tropical countries. 



In conclusion, I ought to mention that for pur- 

 poses of classification Diptera are divided into 

 sixty families. Mr. Verrall, the English authority 



Fig. 8. Middle leg of same FI3' much magnified, showing spikes- 

 for holding and the pointed tibia used in killing prey. 



on this order, has one more, Limnobidae. These 

 families are again subdivided into two large groups, 

 depending for distinction on the nature of the 

 pupae-cases. Roughly speaking, the first group 

 consists of Tipulaoleraceae, Tachydrominae, and 

 Dolichopodidae, the gnats and 

 their relatives, typified by the 

 crane fly ; the second group con- 

 sists of Calliphora erythroceplmla, 

 the blow-fly and its relatives, typi- 

 fied by the larger house-fly, Musca 

 domestica, Sipliona genioulata, the 

 Syrphidae, and ScatopJiaga ster- 

 coraria. 



From the first group, known as 

 the Orthorrhapha, Professor Packard has deducted 

 the fleas or Pulicidae, placing them in an order by 

 themselves. The older observers considered them 

 degraded flies, like the bee-louse, Br aula caeca ^ 

 and Ixodes reduvlus, the sheep tick, which belong 

 to the second group, the Cyclorhapha. 



90 Belsize B.oad, London. 



B 2 



Fig. 9. Teeth of 

 Fly - DoUchopus. 



