THE METAMORPHOSIS OF INSECTS 193 



This discussion illustrates the difficulty of attempting to make sharp distinc- 

 tions, whereas in nature all gradations exist between different types of structure 

 and of development. Thus Leydig ('67) found a certain aphid to be both ovipar- 

 ous and vivaprous; the eggs and the individuals born as nymphs being produced 

 from neighboring tubes of the same ovary. 



Viviparity with sexual reproduction. Although most insects that 

 reproduce sexually are oviparous, there are a considerable number in 

 which sexual reproduction is associated with viviparity. 



Among these sexual viviparous insects there exist great differences 

 in method of reproduction; with some the young are born in a very 

 immature stage of development, a stage corresponding to that in 

 which the young of oviparous insects emerge from the egg; while with 

 others the young attain an advanced stage of development within the 

 body of the mother. 



Sexual viviparous insects giving birth to nymphs or larva. That 

 type of viviparity in which sexual females give birth to very immature 

 nymphs or larvae exists in more or less isolated members of widely 

 separated groups of insects. As the assumption of this type of repro- 

 duction involves no change in the structure of the parent, but merely 

 a precocious hatching of the egg, it is not strange that it has arisen 

 sporadically and many times. In some cases, however, the change is 

 not so slight as the foregoing statement would imply; as, for example, 

 in the case of the viviparous cockroach, which does not secrete 

 oothecae as do other cockroaches. 



Among the recorded examples of this type of viviparity are repre- 

 sentatives of the Ephemerida, Orthoptera, Hemiptera, Lepidoptera, 

 Coleoptera, Strepsiptera, and Diptera. 



Sexual viviparous insects giving birth to old larvce. The mode of 

 reproduction exhibited by these insects is doubtless the most excep- 

 tional that occurs in the Hexapoda, involving, as it does, very import- 

 ant changes in the structure of the reproductive organs of the 

 females. 



With these insects the larvae reach maturity within the body of the 

 parent, undergoing what is analogous to an intra-uterine development, 

 and are born as full-grown larvae. This involves the secretion of a 

 "milk" for the nourishment of the young. 



This mode of reproduction is characteristic of a group of flies, 

 including several families, and known as the Pupipara. This name 

 was suggested for this group by the old belief that the young are born 

 as pupae; but it has been found that the change to pupa does not take 

 place till after the birth of the larva. 



