THE BOT-FLIES. 557 



The DRONE-FLY {EristnHs fcnax) belongs to this family. This insect bears a wonder- 

 ful 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 en- 

 able 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 Muscidas, or Flies, are members of this order, and as at least 

 eight hundred British species of this one family are known, it may be imagined that 

 no description of them can be attempted. 



Ox the right hand of the engraving may be seen a large and bold looking fly. This 

 belongs to the family of the CEstridre, and is popularly known by the name of BOT-FLY. 

 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 aggluti- 

 nated hairs. The eggs are thus conveyed to the stomach, to the coats of which organ 

 the larvae 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 under- 

 ground until they have undergone their metamorphoses, 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 injurious. 



Another kind of Bot-fly ((Es frits boris) 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. \Yhen 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 conducted through curiously-constructed vessels to every part of the 

 body, even to the extremities of the feet and antennae. It 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 would be unable to move a 

 limb. Another characteristic, however, is required. They must be always kept sufficiently 

 open for the free passage of air, and it is not easy to see how these qualities should be 

 united, as a flexible tube will mostly, if abruptly bent, as is continually the case with the 

 air-tubes of the limbs, lose its roundness at the angle, and shut off the communi- 

 cation. An india-rubber gas-tube is a familiar instance of this property of flexible 

 tubes. 



The difficulty is, however, surmounted by a simple and yet most effectual plan. The 

 tubes are double, one within another, and in the interspace a fine but very strong hair- 

 like thread is closely wound in a spiral. It will be seen that, by means of this struct- 

 ure, the tube can be bent in any direction without losing its roundness. The long 

 flexible tubes of Turkish pipes are made in a similar manner, a spiral wire forming the 

 basis, upon which is sewn the leather and silken outer tube one of the many instances 

 where the art of man has been anticipated in the animal creation. A third species 

 (CEstrns OTIS') is parasitic in the sheep, inhabiting the frontal sinus, i. e. the open space 

 between the bones on the forehead and between the eyes. 



