OF GENERATION. 341 



The flesh flies exhibit another instance of ovoviviparous production in 

 insects. It is well known that these flies (Sarcophagte) deposit their 

 larvae upon putrifying flesh, and the young immediately after their 

 birth proceed with the removal of the substance upon which they were 

 deposited. According to Reaumur*, who has described and figured 

 the ovary, the larvae may be found in the spirally twisted egg tube, and 

 which, we may remark incidentally, according to him contains more 

 than twenty thousand larvae. According to De Geer t, the eggs first 

 descend the egg duct after their development at the base of the egg 

 tube is completed, and each ovary contains but from fifty to eighty 

 germs. Their increase is nevertheless very rapid, for in from eight to 

 ten days the larva is grown, and again after eighteen or twenty days 

 the fly appears. If we admit merely the smallest number of eggs, and 

 allow four weeks to the development of every individual, we find, upon 

 supposing an equality of both sexes in each generation, in one summer 

 (from June to October) a produce of more than five hundred millions, 

 therefore about half as many individuals as there are human beings 

 upon the whole earth, according to the received opinion. Meantime, 

 how many are destroyed as larvae by their multitudes of enemies ? how 

 many also as flies are there not consumed by birds ? 



Similar cases of an early exclusion from the egg within the body of 

 the mother has been observed in other genera. Reaumur ^ found the 

 larvae of a small Tipula, which, to judge from his figure, apparently 

 belongs to Meigen's genus Ceratopogon, in one of his boxes, where also 

 they changed into nymphae. He obtained from these the fly which 

 subsequently produced long worm-shaped larvae ; indeed, upon a slight 

 pressure, he squeezed them fully developed from the body of the 

 mother. According to Kirby and Spence also many Cocci and bugs 

 bring forth living young ones; the latter from the observation of 

 Busch, upon which, however, I have not been able to obtain more 

 detailed particulars. 



But we have, more positive observation upon the development of 

 the Diptera pupipara. The remarkable form of the ovary of the 

 female is shortly indicated above ( 136. III. 2.). The egg descends 

 from the small ovary through the egg duct into the large, bag-shaped, 



* M&noires, &c., vol. iv. part ii. p. 153. PI. XXIV. f. 1. Edit, in 12mo. 



f Ib. vol. vi. p. 31. PI. III. f. 518. 



+ Ib. vol. iv. partii. p 168. PI. XXIX. f. 1015. 



Introd. to Entom. vol. iii. 



