﻿40 BULLETIJSr 136&, U. S. DEPARTMENT OF AGEICULTUEE 



On January 4, 1924, 21 larvse from 11 ta 16 millimeters in length, 

 from gullets, were introduced subcutaneously above the hock of 

 another cow. On January 8 one larva cut through the skin 5i/^ 

 inches above the incision. On the following day 5 others punctured 

 the skin, one of these being 30 inches above the point of introduc- 

 tion. The following day 3 others appeared, 1 of these being near 

 the spinal column on the right side about 7 inches behind the shoul- 

 der, a distance of 40 inches in a line from the incision. A total of 

 14 grubs perforated the skin, all being on the right side. 



It appears from these tests that when larv?e which are ready 

 to leave the gullet are introduced subcutaneously they may not 

 travel far from the point of introduction before puncturing the 

 skin, and that they appear to pass upward under the subdermal con- 

 nective tissue without penetrating deeply into the tissues as oc- 

 curs when young larvse from the gullet are introduced. 



To recapitulate, in the tests carried out at the Dallas laboratory, 

 28 animals were infested by allowing flies to oviposit on the legs 

 and bodies and were prevented from reaching any part of their 

 bodies with tongue or mouth. Of these 28 animals, 2 were acci- 

 dentally killed and not examined. In one of those slaughtered 

 for dissection no larvas were found, but in this case nO' hatched 

 eggs were observed on the animal and other evidence indicated 

 that none of the eggs placed upon it were viable. One of the other 

 animals was killed immediately after the eggs hatched and, al- 

 though no larva3 were found, there were strong indications of the 

 penetration of larva3 through the skin. Two of the other animals 

 killed showed an abundance of grubs along the gullet and on the 

 viscera. Twenty of these animals were kept for observation during 

 the fall and winter, and in every one of them grubs came to the 

 back during the normal grub season. In all of the 11 cattle which 

 were infested by introducing larvae from the gullets of slaughtered 

 cattle under the skin on the legs, behind the ear, and on the back, 

 larva3 appeared under the skin at the usual time during the fol- 

 lowing fall and winter. Lar\^£e were also recovered in 2 animals 

 which were dissected after larvae had been introduced under the 

 skin in this way. On the other hand, among the 8 animals to 

 which larva3 and eggs were given by mouth, neither of the 2 which 

 were carefully dissected showed the presence of larvae, and in 

 not a single instance among the 6 cattle held for observation did 

 a grub appear along the back. 



In 1922 Carpenter, Phibbs, and Slattery {19) report similar 

 results in experiments carried out in Ireland. No grubs were 

 recovered in calves to which numerous larvae were fed but heavy 

 infestations developed in carefully muzzled calves exposed to the 

 oviposition of flies. 



The fact that the larva3 penetrate through the skin is established 

 by the following evidence : 



1. — The eggs are usually laid on the hairs comparatively close 

 to the skin, which would favor penetration. They are firmly at- 

 tached and the eggshells, after the larvae have escaped, remain at- 

 tached to the hair. 



2. — The eggs are not fitted with an operculum as in the case 

 of the horse bots, the ends merely splitting as the larvae emerge. 



