446 THE INVERTEBRATA 



Certain larvae show an even more complete adaptation to life in water 

 in that though they possess a tracheal system this is entirely closed 

 from the exterior and in their early stages it is filled with fluid. Such 

 forms respire of necessity by a process of simple diffusion through the 

 general integument, e.g. Chironomus and Simulium. 



Reproduction. The sexes of insects are separate, leery a purchasiy 

 a remarkable exception, being the only known self-fertilizing her- 

 maphrodite in the class. The usual method of reproduction is by de- 

 position of yolky eggs following copulation. The egg, except in many 

 parasitic Hymenoptera, is richly supplied with yolk and invested with 

 a vitelline membrane and further protected by a hard shell or chorion. 

 The chorion exhibits different degrees of external sculpture and it is 

 perforated at some point or points to allow of sperm penetration. 

 The spermatozoa, which are of the filiform type, may be transmitted 

 to the female in the form of a spermatophore. Though insects are 

 on the whole prolific creatures capable of producing large numbers 

 of eggs, a few cases are met with where females only lay a few eggs 

 in the course of their life. Thus, in the viviparous tsetse flies, a single 

 egg is passed to the uterus about every nine or ten days. The larva is 

 there nourished by special "milk" glands till it is fully fed when it 

 is passed out for immediate pupation. Viviparity and reduced tgg 

 production are here obviously associated with one another. In a large 

 number of cases reproduction is effected without the intervention of 

 the male. This phenomenon of parthenogenesis is best seen in the 

 aphides or plant lice where several generations resulting in the pro- 

 duction of parthenogenetic females are passed through. The racial 

 advantage accruing from this greatly increased reproductive capacity 

 is obvious. 



Parthenogenesis is in certain cases, e.g. among the family Cecido- 

 myidae of the order Diptera, found to occur in larval forms. In 

 Miastor, a form living in decaying wood and bark, reproduction in 

 this manner (paedogenesis) occurs for the greater part of the year. These 

 larvae contain prematurely-developed ovaries from which as many as 

 thirty larvae may grow. In summer, larvae occur which are morpho- 

 logically different from the paedogenetic forms. These summer larvae 

 pupate and the small midge-like flies which emerge lay four or 

 five large eggs; from these a further series of paedogenetic larvae 

 arises. 



Among a few of the parasitic Hymenoptera, e.g. some Chalcididae, 

 the phenomenon oi polyembryony has been observed. This consists in 

 the development of more than one embryo from a single Qgg. In 

 Copidosoma gelechiae, which parasitizes a caterpillar living on the 

 goldenrod Solidago, a hundred or more embryos may result from the 

 deposition of a single egg. 



