﻿54 BULLETIN 1369, U. S. DEPARTMENT OF AGEICULTUEE 



immediately after the larvse reach it, and never more than three 

 days later. Usually the first indication of a break in the skin is 

 in the nature of a minute, irregular hole which is usually accom- 

 panied by slight bleeding. Within a day or two after the first break 

 is made the edges of the hole become more smooth and round. In 

 several instances the writers have obser\^ed a circular plug of skin 

 about 2 millimeters in diameter and bearing hair, seated in a freshly 

 cut hole. In two cases these plugs were still attached at one side as 

 though the cutting process was not complete. 



The only evidence to indicate which end of the larva does the 

 cutting is that in every case where larvae have been extracted the 

 posterior end is always outward. This has been found even before 

 the cutting of the hole was completed. It is conceivable that the 

 larvae may do the major part of the cutting with the mouth parts 

 and then turn around and complete the enlarging of the hole with 

 the posterior end. Wlien these early stages are removed by pres- 

 sure they appear to be in a position perpendicular to the skin rather 

 than lying horizontally under it as is the normal position in sub- 

 sequent stages. 



DEVELOPMENT OF THIRD-STAGE LARV^ 



The first molt under the skin, from the third to the fourth stage, 

 takes place soon after the hole is completed. The determina- 

 tion of this period with exactness is very difficult, but the records 

 of the writers show that the molt in H. lineatum may take place in 

 about 24 hours after the hole is completed, or the stage may last 

 from 6 to 8 days. The average period, based on 17 records made 

 at Dallas, Tex., with considerable exactness, is 4.5 days, and the 

 average period based on 13 records made in New York is 4.26 

 days. The period from the cutting of the hole in the skin to the 

 molt in H. hovis, as observed in New York, ranged from 1 to 6 

 days with an average of 3.35 days. 



For several hours prior to the molt the spiracles of the fourth 

 stage can be seen beneath the skin of the larvae. The exuvia are 

 very delicate, but seem to be shed almost in their entirety at one 

 time. The break in the larval skin is very irregular. The posterior 

 ends of the tracheal tube where they join the spiracles are rather 

 highly chitinized and they are shed along with the skin. 



The writers' observations show that not infrequently the third- 

 stage larva may move to a new location under the skin, usually 

 within a few inches of the first, and cut a second hole or even a 

 third hole before it molts. 



ENCYSTMENT AND DEVELOPMENT OF FOURTH-STAGE LARV^ 



It appears that about simultaneously with the molt to the fourth 

 stage the tissues of the host change their form of resistance from 

 an active leucocytosis to a segregation of the insects by the forma- 

 tion of encystment sacs (fig. 26). With the development of these 

 pouches the general inflammation of the connective tissue subsides. 

 The skin of the third stage remains in the sac, which is formed 

 very rapidly, and gradually the walls of the sac become thicker as 

 the growth of the larva continues. During this and all subse- 

 quent stages the larva remains with its posterior spiracles toward 



