146 INSECT TRANSFORMATION 



through the period of embryonic growth 1 and the organs, 

 sexually differentiated, being visible through the transparent 

 body- wall of the aquatic larva or " blood-worm ", in the later 

 stages of which the genital ducts also may be seen. 2 These 

 latter arise in moths and butterflies from strands of tissue, 

 known as the genital cords, connecting the testes or ovaries 

 with the ventral body-wall. External reproductive structures, 

 however, such as the ovipositor as well as the terminal median 

 chitin-lined tube the vagina of the female or the ejaculatory 

 duct of the male are present in the larva only as inwardly- 

 growing imaginal buds, 3 which during the pupal stage become 

 developed into the adult condition. Meanwhile by the 

 multiplication and modification of germ-cells the rudimentary 

 ovary shows increasing differentiation of the ovarian tubes, 

 with their delicate epithelial walls and contained eggs, while 

 the testis also grows rapidly, its cavity becoming lobulated, 

 and the contained immature male germ-cells (spermatogonia) 

 dividing so as to give rise ultimately to the active mobile 

 sperm-cells (spermatozoa). Thus, quickly after emergence 

 from the pupal cuticle, the winged adult insect may be ready 

 for the breeding function. 



At this point it may be well to mention some very curious 

 cases of abnormal hastening of the development of the eggs 

 in certain insects which become capable of reproduction 

 before they have completed their transformations. The midge 

 Chironomus has just been mentioned, and in one species, at 

 any rate, it was observed fifty years ago 4 that the female pupa 

 may lay eggs which, without fertilization develop into larvae. 

 This may clearly be regarded as a case of precocious virgin 

 reproduction (parthenogenesis), and we have seen that egg- 

 laying or the birth of active young from virgin females is a 

 common feature in the life-cycle of most kinds of greenfly 

 (Aphididae). It had previously been noticed 5 that among 



1 R. Hitter : " Die Entwicklung der Geschlechtsorgane und des Darmes 

 bei Chironomus ". Zeitschr. f. wissensch. Zool. LI. 1890. 



2 L. C. Miall and A. R. Hammond : " The Structure and Life-history of 

 the Harlequin Fly ". Oxford, 1900. 



3 W. Hatchett Jackson : " Studies in the Morphology of the 

 Lepidoptera ". Trans. Linn. Soc., Vol. V. 1890. 



4 O. Grimm : " Die ungeschlechtliche Fortpflanzung einen Chironomus " 

 Mem. Acad. St. Petersburg (7), XV. 1870. 



6 N. Wagner : " Ueber die viviparen Gallmiickenlarven ". Zeitsch. f. 

 wissensch. ZooL, XV. 1865. 



