GENERATION OF INSECTS. 421 



nervous chord, and the dorsal vessel, are laid. An attenuated pos- 

 terior prolongation of the ellipsoid vitelline or alimentary sac forms 

 the rectum, and opens upon the thirteenth segment. 



In such a condition, but without the cephalic and trophal develop- 

 ments, the entozoiform larva of the flesh-fly is bom or excluded from 

 the parent : in a similar condition the larva of the bee and of the 

 parasitic Hymenoptera quits the vermiform ovum, but without the 

 external communication with the digestive or vitelline sac having 

 been established at the posterior extremity. 



In some Coleoptera development proceeds to the formation of the 

 appendages of the head, as above described, and a capitate but 

 apodal larva is excluded, as in the nut-weevil. 



In other Coleoptera, as the DonacicR, the ventral arcs of the 

 second, third, and fourth segments send out bulbous rudiments of 

 the thoracic legs, before the tergal or notal elements of the segments 

 are completed ; the abdomen is closed above, whilst the development 

 of the extremities has proceeded to the formation of obscure joints 

 and terminal hooks. The rudimental palpi begin to bud from the 

 maxillae and labium ; the mandibles acquire their hard terminal 

 hooks, and closely resemble the thoracic feet. In this state the larva 

 is excluded. 



At an earlier period the simple bulbous antennae, mandibles, and 

 maxillae, indicate three cephalic segments, equal in size and dis- 

 tinctness to those of the thorax. The maxillary palpi, the labrum 

 and labium, might perhaps be regarded as indicative of three other 

 abortive segments ; and if, according to the analogy of the Crustacea, 

 the eyes are to be regarded as appendages of a proper segment, 

 then seven cephalic segments may be reckoned, although three only 

 can be defined by observation of the early development of the insect. 

 The malpighian and other tubular glands result from juxtaposition in 

 a linear series of derivative nucleated germ-cells, which coalesce by 

 liquefaction of the parts of the cell-wall in contact with each other, 

 the nuclei remaining longer and indicating the primitive separation 

 of the cells. The ovarian tubes have appeared to me, in the larva 

 of the silkworm, to retain the primitive series of nuclei of the germ- 

 cells at their capillary beginnings ; whilst coalescence of the germ- 

 cells themselves, has taken place to form the lower part of the tube : 

 such persistent, primitive, nuclei, or granules, seem to form the basis 

 for the formation of the subsequent ova. 



The further progress in the development of the Insect cannot be 

 better entered upon than in the words of our celebrated entomolo- 

 gists, Kirby and Spence, to whom we owe the most useful and popular 

 introduction to their delightful science. They say : — 



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