DIPTERA. — STRIDE. 579 
skin of the grub becoming a cocoon for the inclosed nymph, as in the 
other insects undergoing a coarctate transition. 
The horse bot-fly deposits its eggs on the hairs of those parts of 
the horse, which can be easily reached by the tongue of the animal, by 
which means the larve are conveyed into the stomach. The larve 
of this species ( jig. 132. 19.) are very strongly armed with short thick 
spines, directed backwards*, and arranged in double transverse rows 
at the base of each segment of the body ; the head is very small ; its 
upper part is produced into two tubercles, which may possibly be ocelli; 
the maxilla, judging them to be such from their situation, are a pair 
of very strong hooks, bent outwards; rather above and between the 
base of these organs is a pair of much smaller slender corneous organs, 
which must be the mandibles, and which do not appear to have been 
previously noticed (fig. 102. 20. head of the larva of G. Equi) in this 
larva. The spiracles are numerous, placed at the posterior part of the 
body, but defended by the folds of the terminal segment. 
Mr. Newport has given a highly magnified figure of the head of the 
larva of C&strus Ovis, which lives in the frontal sinus of the sheep (art. 
Insect in Cycl. of Anat. p. 23.), in which the front of the head is simi- 
larly produced, but each side is furnished with two minute ocelli ; 
towards the front of the head are the horny mandibles ?, much longer 
than in GE. Equi, hook-like, and directed backwards, whilst the max- 
ille ? are still smaller hooks, directed inwards. ‘This larva is destitute 
of the recurved spines; it is furnished with two spiracles at the ex- 
tremity of the body, which are defended from injury in the same man- 
ner as in the horse bot. 
In the larvee of CEstrus Bovis,which reside in the large open tumours 
of the backs of horned cattle, we find a double modification of struc- 
ture, admirably in accordance with their habits. Residing immov- 
ably ina 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 subject 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 tumour, it is in this part of 
the insect that the two large principal spiracles are found. When 
full grown, they push themselves backwards out of the tumours, and 
* We have seen in many instances that spines of this kind are employed in 
progression; hence their importance in enabling the larva to make its way out of 
the stomach of the horse to the earth, when full grown. 
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