686 TEXT-BOOK OF ENTOMOLOGY 



organs which remain functional during the metamorphosis are immune from the 

 attacks of the phagocytes. The heart in the muscids continues to beat, as 

 Ktinckel d'Herculais has observed, during the entire period of the metamor- 

 phosis, with the exception of a day or two in the latter half of it. The nervous 

 system must continue functional during the entire time. The three pairs of 

 thoracic muscles which pass intact from the larva to the imago are probably 

 employed in respiration during the metamorphosis. The reproductive glands 

 are, like the imaginal disks, rapidly growing organs." He adds that among the 

 other holometabolic insects many or most of the larval organs remain functional 

 during metamorphosis, hence there is but little histolysis. "But the larval 

 intestine would always necessarily become unfunctional, and, as we have seen, 

 Kowalevsky is of the opinion that the larval mid-gut in all holometabolic insects 

 contains imaginal disks, and undergoes degeneration during the metamorphosis." 



The post-embryonic changes and imaginal buds in the Pupipara 

 (Melophagus). The sheep-tick (Melophagus) is still more modified 

 than the Muscidse ; the larva is apodous and acephalous, but, as 

 Pratt observes, much less highly specialized than those of muscids, 

 and in respect to the position of the thoracic buds it closely re- 

 sembles Corethra. They lie just beneath the hypodermis in 

 two very regular rows, and not in the centre of the body, as in 

 Musca (Figs. 628, 636, C). While, however, in Corethra all the tho- 

 racic buds are of larval origin, arising after the last larval moult, in 

 Melophagus, on the other hand, each of these buds, except the 

 dorsal prothoracic, arises in the embryo, as is also the case in Musca. 



In the cephalic buds the conditions are similar to those in Musca, 

 but still more complicated. Instead of a single pair of head-buds, 

 there are two pairs, one dorsal and one ventral. " The dorsal pair 

 corresponds to the muscidian head-disks in every respect ; they are 

 destined to form the dorsal and lateral portions of the imaginal 

 head, together with the compound eyes. The ventral head-disks 

 have no counterpart in Musca. The fate of these disks or buds is to 

 form the ventral portion of the head, the paired projections forming 

 the rudiments of the proboscis. 



" The formation of the head-vesicle proceeds in a way similar to 

 that in Musca. The ventral disk fuses early at its lateral edges with 

 the dorsal pair. The communications between both ventral and 

 dorsal disks and the pharynx rapidly widen (in the old larva they 

 have already become very large), and soon the disks and pharynx 

 form together a single vesicle, which is the head-vesicle." The 

 imaginal buds of the abdomen Pratt finds to be exactly as in the 

 corresponding ones of Musca. 



In the embryo of Melophagus the cephalic and thoracic imaginal 

 buds first appear as local thickenings, followed by the invagination 

 of the ectoderm ; the cephalic buds first appear very early in the 



