436 THE BOT-FLY. 
the Tabanidee. As with the preceding family, the larvee of the Asili reside under ground, 
and feed upon the roots of plants. 
The family of the Syrphidee, or Hoverer-flies, is rather large, and contains many inter- 
esting insects. Among them may be mentioned the Volucella flies, which feed, while in the 
larval state, on the larvee of bees and wasps, and, as if to aid them in gaining admission into 
the nests of those formidable creatures, are shaped and colored so like the insects which they 
invade, that at a little distance it is almost impossible to distinguish between them. 
The Drone-FLy (Hristalis tenaz) belongs to this family. This insect bears a wonderful 
resemblance to the hive-bee, and has a habit of moving the abdomen in a manner that leads an 
unaccustomed observer to fancy that it possesses a sting. The larva of this insect is popularly 
known by the name of Rat-tail maggot, on account of its peculiar construction. This larva 
resides in mud, with the head downwards. In order to enable it to breathe, the respiratory 
tubes are carried into a long and telescopic appendage attached to the tail, the end of which is 
furnished with a brush of hairs something like that on the tail of the gnat larva. The 
extremity of this curious organ is always held out of the muddy water, and it is most curious 
to see the grubs elongate their tails as the depth of water is increased. 
All the vast family of Muscidee, or Flies, are members of this order, and as at least eight 
hundred species of this one family are known, it may be imagined that no description of 
them can be attempted. 
THE large and bold looking fly, represented in our illustration, belongs to the family of the 
(Estridee, and is popularly known by the name of Bor-riy. All these insects are parasitic in 
or upon animals. The larva of this Bot-fly resides in the interior of horses, and is conveyed 
there in a very curious manner. The parent fly deposits her eggs upon the hairs near the 
shoulders of the horse, where the animal is sure to lick them in order to rid itself of the 
unpleasant feeling caused by agglutinated hairs. The eggs are thus conveyed to the stomach, 
to the coats of which organ the larvee cling, and there remain until they have attained their 
full growth. They then loosen their hold, are carried, together with the food, through the 
interior of the animal, fall to the 
ground, and immediately begin to 
burrow. They remain underground 
until they have undergone their meta- 
morphoses, and then emerge in the 
shape of the perfect insect. They do 
not seem to inflict any damage upon 
the animal from whose bodies they 
have drawn their nourishment, and 
some veterinary surgeons believe that 
they are rather beneficial than in- 
jurious. 
Another kind of Bot-fly (@strus 
bovis) resides in the cow, but instead of being taken into the stomach, it burrows into the 
skin, and there forms large tubercles, that are popularly called worbles or wurbles. An 
aperture is always left on the top of the tubercle, and the larva breathes by means of 
keeping the two principal spiracles opposite to the orifice. When full grown, they push 
themselves out of the aperture, fall upon the ground, and there burrow and undergo their 
transformations. 
The spiracles, to which allusion has been often made, are the apertures through which air 
is admitted to the system. Insects breathe in a very remarkable manner, the air being con- 
ducted through curiously-constructed vessels to every part of the body, even to the extremities 
of the feet and antenna. Tt will be seen that the structure of these vessels must be very 
remarkable, on account of the opposite duties they have to perform. As they penetrate the 
entire insect, it is needful that they should be flexible. in order to permit the creature to move 
about at will, as, if they were stiff-walled, the joints would be rendered useless, and the insect 

HORSE BOT-FLY.— Gastrophilus equi. a, Fly. b, Egg ona hair. c, d, e, Larva in 
its transformations. 
