38 



that the fly does not pierce the skin of cattle with its ovipositor at all, but merely glues 

 its eggs to the hairs, while the grubs, when hatched, eat their way under the skin ; while 

 Eeaumur asserts, on the contrary, that the mother fly deposits her eggs in the flesh itself. 

 At all events, the grubs are found in large open tumours on the backs of horned 

 cattle, making for themselves, says E-eaumur, " a place where food is found in 

 abundance, where they are protected from the weather, where they enjoy at all times an 

 equal degree of warmth, and wliere they finally attain maturity." Those parts of the 

 animal's body in which the larvse are lodged can be easily recognized, as above each larva 

 can be seen a tumour or bump, which has been, not inaptly, compared to the swelling pro- 

 duced on the forehead by a smart blow. 



In these larvae we find a double modification of structure admirably in accordance with 

 their habits. Residing immoveably in a fixed spot, they do not require the strong mouth 

 hooks which the horse bot employs to retain it in its station in the stomach, where it is, of 

 course, subjected to a variety of action, the parts of their mouths are therefore soft and 

 fleshy : on the other hand, the extremity of the body being exposed at the orifice of the tu- 

 mour, it is in this part of the insect that the large spiracles or breathing pores are found. It 

 is, therefore, very essential to the grub that the hole of the tumour should remain constantly 

 open, for by this aperture a communication with the air necessary for respiration is pre- 

 served, and the grub is thence placed in the most favourable position for receiving air. 



It is commonly on young cattle of two or three years old that these tumours are 

 found, it being very rare to find them on very old animals. 



The larvae when young are white, but become brown by degrees, attaining at maturity 

 a very deep colour. They are furnished with transverse rows of minute hooks, which 

 are probably used in moving about, and are, doubtless, a source of great irritation. The 

 larva when mature is about an inch long. The bumps are scarcely perceptible before the 

 beginning of winter, and the larvae live in them during the entire winter. 



Reaumur tried to discover how the larva, when arrived at its full growth, succeeds 

 in leaving its abode, for the opening of the tumour is smaller than its body. 



" Nature," says Reaumur " has taught this worm the surest, the gentlest, and the 

 most simple of methods, the one to which surgeons often have recourse to hold wounds 

 open or to enlarge them. They press tejits into a wound they wish to enlarge. Two or 

 three days before the worm wishes to come out, it commences to make use of its posterior 

 part as a tent to increase the size of the exit from its habitation. It thrusts it into the 

 hole and draws it out again many times in the course of two or three days, and the oftener 

 this is repeated, the longer it is able to retain its posterior end in the opening, as the hole 

 becomes larger. On the day preceding that on which the worm is to come out, the pos- 

 terior part is to be found almost continually in the hole. At last it comes out backwards 

 and falls to the ground, when it gets under a stone or buries itself in the turf, remaining 

 quiet, and preparing for its last transformation. The skin hardens, the rings disappear, 

 and it becomes black. Thenceforth the insect is detached from the outer skin which 

 forms a cocoon or box. At the front and upper part of the cocoon is a triangular piece 

 which the fly gets rid of when it is in a fit state to come into the open air." 



Fig. 30, taken from Reaumur's drawings, represents the 

 fly emerging from its cocoon. The ovipositor or instrument by 

 which the eggs are laid is also shown. This instrument, which 

 is attached to the anus of the female, is a tube composed of four 

 pieces which, like the joints of a telescope, are retractile within 

 each other. 



Reaumur, whose theory is, as we have seen, that the fly 

 pierces the flesh when depositing her eggs, states that the act is 

 not attended with much pain unless some very sensible fibres 

 are touched, 



" It ought to be remarked," says Rennie, from whom we 

 quote, "that cattle have very thick hides, which are so far from 

 being acutely sensitive of pain that in countries where they are 

 put to draw ploughs and waggons they find a whip ineff'ectual to 

 , drive them, and have to use a eroad in form of an iron needle at 



^.60. Imago of bot fly emerg- , , , „' . , ,o . . n- . -i ^ l^ -Li-Zi 



ing, and ovipositor oi female, the end of a stick. Were the pain inflicted by the bot-nyvery 



