THE PUPIPAROUS HABIT 139 



here. Their peculiar method of reproduction is the outcome of the cir- 

 cumstances under which the larval stage is passed, for these flies deposit 

 their eggs, or young larvae just hatched from the eggs, 

 in the bodies of other larger larvae, usually those pupiparou8 habit 

 of Lepidoptera, and a reduction of the immature 

 stages is a necessary condition of their existence. The larval stage 

 must be completed before the host pupates, or immediately after, lest 

 the food supply should be exhausted. In the other two cases there is no 

 such apparent necessity for an acceleration of the stages, and we must 

 look elsew r here for an explanation of the origin of the habit. Probably 

 two factors enter into it. In the first place, the larva while retained in 

 the body of the active female will avoid a large proportion of the dangers 

 to which a free larva is subjected, both from predaceous insects and 

 from birds, and from accidental conditions such as loss of food 

 supply, exposure to adverse conditions of temperature and moisture, etc. 

 A larva which only leaves the body of the female when it is ready to pro- 

 tect itself by pupation has obviously a better change of reaching the 

 mature stage than, one which has to live free for a considerable period. 

 Secondly, the rich supply of food which blood-sucking insects can obtain 

 appears to have some relation to the pupiparous habit. Although the 

 number of Diptera which feed on blood is exceedingly small in propor- 

 tion to the size of the order, the number of pupiparous forms is very high, 

 including the whole group known as the Pupipara, the genus Glossina, 

 and some Muscids. Even in Philaematomyia insignis a shortening of the 

 first stage can be noticed, for the eggs are much larger when laid than 

 one would expect for the size of the fly, and the larvae hatch from them 

 at a much earlier period than the larvae of other Muscids under iden- 

 tical conditions. The subject is a very obscure one, but the association 

 of the blood-sucking habit with the pupiparous habit is clear enough, and 

 one should always be on the look out for larviparous forms among those 

 known to suck blood, whether they are biting flies or not. 



The term ' pupiparous ' is really a misnomer, for the instar which is 

 produced from the mature female is a full-grown larva, which does not 

 pupate until it is in suitable surroundings. The larva of Glossina when 

 born is able to move about until it finds a suitable place for pupation. 

 That of Hippobosca is not motile, but passes into the pupal stage at the 

 place where it leaves the female. 



Musca bezzii is an example of the second degree, in which a young larva 

 is produced. In the early part of its life the larva is nourished by the 

 mother, while in its later stages it obtains food for itself after the 



