Apbii. 2, 1894.] 



KNOWLEDGE. 



79 



times mistaken for it. The true cattle pest may, however, 

 be distinguished by its comparatively gigantic size ; it 

 attains a length of about an inch, and has an expanse of 

 wing of at least two inches, and this is a very large size 

 for a British fly. It flies with a loud hum, and wheels 

 round its prey in large circles before commencing the 

 attack. The female lays some four hundred or five 

 hundred eggs, which are deposited on grass stems, and 

 the larvfe when hatched wriggle down and work thenselves 

 beneath the soil, where they soon find appropriate food. 

 A degree of fecimdity such as this may well cause this 

 insect to become an intolerable nuisance in any district in 

 which it is well-established, not only to cattle, but to human 

 beings as well, for it is quite ready to attack mankind. 

 Fortunately it is by no means common in this coimtry, 

 and we are therefore tolerably free from molestation. 



But even the gadflies have had their apologists. In the 

 days when the "apothecary's ' chief instrument was the 

 lancet, and blood-letting was regarded as the universal 

 panacea for human ills, it is scarcely surprising that the 

 idea was entertained that the attacks of gadflies on cattle 

 were not altogether an unmixed evil, but that in-fact, by 

 their phlebotomy, these insects might even be a valuable 

 preventive of disease in full-fed animals, and that their 

 incessant attacks, by keeping the cattle in constant move- 

 ment, were useful in giving them needed exercise, and 

 thus holding in check such diseases as might be generated 

 by indolence and repletion. 



In this connection may be mentioned that terrible pest 

 of Central Africa, the tsetse fly. Though such a fearful 

 scourge, it is but a small insect far inferior to the gadfly 

 in size, and indeed not much larger than an ordinary 

 house-fly. According to Livingstone, its attack produces 

 no immediate eft'ect on the cattle, and symptoms of con- 

 stitutional derangement do not appear till a few days after 

 the puncture has been made. The eyes and nose then 

 begin to run, and swellings appear under the jaws ; 

 though the animal continues to graze, it becomes thin and 

 starved in appearance, and its flesh becomes flaccid and 

 feeble. Some perish at an early stage with staggering and 

 blindness, as though the brain were affected ; others linger 

 on, becoming more and more emaciated, until at last they 

 die of extreme exhaustion, all the organs of the body being 

 more or less diseased. 



There is a most remarkable section of the fly order, 

 which contains flies parasitic upon v.'irious mammals and 

 birds ; some of them, like the bat-louse and sheep-tick, 

 have no wings, and their popular names reflect the 

 mistakes that have thence arisen as to their zoological 

 position. Though their habits are those of lice and ticks, 

 they are yet true flies — very extraordinary ones, it is 

 true, but still none the less members of the order 

 Diptera though they have no wings. Some near relations 

 of theirs may be appealed to in support of this contention ; 

 the bird-louse or fly (Ornithomijia) and the horse-louse or 

 fly {Hippobosca) are in shape of body and general habits 

 very much like the sheep-tick, but they have the usual pair 

 of wings of dipterous insects. These insects live upon the 

 blood of their hosts, which they suck up through a fleshy 

 tube leading into the mouth, composed of the labium and 

 maxiUse and provided with piercing bristles. On the death 

 of their host they forsake it at once, and proceed to seek 

 for another ; when on this search the bird-louse is not always 

 particular as to the animal from which it shall elect to 

 take its next draught, and if unlucky man should fall in 

 the way he will become the victim. Hippohosca equina is 

 a great pest to horses, settling on them as they pass 

 through woods, and running in spirited fashion over 

 their bodies in search of those parts that are least covered 



with hair. These found, they plunge theii- straight beak 

 through the skin and greatly u-ritate the quadruped. 

 Swishing with the tail, the horse's only method of dislodg- 

 ment, is not very eflective, because the flies cling so tightly, 

 and have such tough bodies. By far the most remarkable 

 facts connected with these insects are those that concern 

 their reproduction. They are not prolific, the female pro- 

 ducing no more than a single egg at a time ; there is, 

 however, a good reason for this slow rate of increase, for 

 the egg is not actually laid when fertilized, but is retained 

 within the body of the mother, where the larva is hatched 

 and remains during the whole of its larval life. During 

 this time it is fed on a milky secretion furnished within 

 the mother's body, and of course it grows a good deal, and 

 the body of the mother, which is sac-like without evident 

 segmentation, expands to accommodate it. Thus there is 

 no room for more than a single egg. As soon as the 

 immured grub has assumed the pupa state, it is deposited 

 by the mother as a large, soft, white, roundish body which, 

 but for its size, might easily pass for an ordinary egg. 

 As we have seen, however, it is really a pupa, or rather 

 puparium, since, according to fly custom, the real pupa is 

 enclosed in the last larval skin. The apparent eggs, then, 

 laid by the flies of this division are not really eggs at aU, 

 but pupae, and hence the group has been named "Pupipara," 

 i.e., bringers forth of pups. 



Brief reference may now be made to the Hemiptera. 

 All of these insects possess four piercing bristles in a 

 gutter-like labium, which constitutes a sort of beak, usually 

 carried, when at rest, bent backwards beneath the head. 

 They aU feed upon liquids, derived either from animals or 

 plants, the latter being by far the most usual source 

 of supply. As a consequence, only a very few species 

 ever trouble mankind, for even amongst those that are 

 animal feeders it is the rarest thing for man to be the 

 animal selected, and indeed it is usually by accident that 

 such an event happens. Of course there is the bed-bug, 

 which seems to have permanently attached itself to man- 

 kind, but beyond this no species can be spoken of as other 

 than quite an occasional and incidental assailant. We 

 have sufficiently discussed our bedroom pest on former 

 occasions, and need now therefore only notice the other 

 species that are likely to trouble us. These are almost 

 entirely to be found amongst water insects, two of which 

 are all that call for notice. The first is the water boatman 

 [Sotonecta ylauca),a, well-known inhabitant of ponds, which 

 swims on its back, oaring its boat-shaped body about with 

 the utmost vigour by means of its flat and fringed hind 

 legs, and every now and then resting at the surface with 

 the tip of its body just out of the water for breathing 

 pui-poses. It is a strong, active, and rapacious insect, 

 wielding an iron rule over the other inhabitants of the 

 pond ; of course it does not eat them, since it has no 

 biting jaws, but it hugs them tight, digs its beak into 

 them, and sucks out their juices. It is a bold and 

 enterprising insect, and very much objects to having its 

 liberties curtailed, so that if caught in a 

 net and then taken into the hands, it is 

 not slow to manifest its indignation by 

 a sharp prick with its beak ( Fig.7.) This, 

 however, can hardly be intended for 

 taking food, but must be regarded as 

 a defensive act. The pain is acute 

 at the moment, but it soon passes oft", 

 and no after ill eS'ects are perceived. 



The other insect is a closely allied 

 species, of a dark olive-brown colour, 

 broad and flat instead of narrow and 

 deep, as the boatman is. It is not so 



Fiff. 7.— Head of 

 Water Boatman, with 

 beak extended, a, an- 

 tennae ; i, beat; e, eves. 

 Magnified three dia- 

 meters. 



