THK ORGANS OK DIGESTION. 149 



therefore, which the nutrimental canal undergoes in these orders 

 consists in its lengthening in proportion to the increasing size of the 

 insect, and at the time of moulting it covers itself internally with a 

 new mucous membrane, the old one being rejected by the anus,, or 

 probably absorbed. This changing of the skin in the intestine is 

 certainly remarkable, and proves, as well as the similar phenomenon in 

 cutaneous affections in man, in which the epidermis peels off (for ex- 

 ample, after scarlet fever), the perfect uniformity of the intestinal mucous 

 membrane with the exterior epidermis. The larvae of the Libellulfs 

 alone appear to make a slight exception to the rule of the intestinal 

 canal remaining the same, their's being somewhat larger, particularly 

 broader, than in the perfect insect, and in the latter the respiration of 

 the colon disappearing, which was peculiar to the former. 



Insects with a perfect metamorphosis, on the contrary, undergo in 

 the intestinal canal, as well as exteriorly, important changes, which, 

 however, refer only to the form, the structure remaining constantly the 

 same. It is true the membranes are originally much more delicate, 

 looser, and admit of being more readily separated, particularly in the 

 stomach, but this difference gradually vanishes. During their larva 

 state the intestine assumes a new skin at every moulting f ; towards 

 the end of this period, and still more during their pupa state, the 

 intestine shrinks, particularly the stomach, and acquires thereby a 

 more compact appearance. It is the divisions of the nutrimental canal 

 and their relative lengths which chiefly vary, but these are regulated by 

 very different laws in the several orders, and consequently demand of 

 us an especial notice. 



The maggots of the Dipt era (PI. XVIII. f. 2. maggot ; f. 3. fly) have 

 a longer intestine than the flies, but it is the stomach chiefly which 

 occasions this greater length. The sucking stomach is present, but 

 larger, more shortly pediculated, and, besides, there are large cylindrical 

 salivary bags, which in the course of their change transform themselves 

 into filiform salivary vessels. The biliary vessels remain uniform both 

 in number and shape. During the larva state the intestinal canal 

 remains unchanged, but it alters the more quickly in the pupa state ; 



* Compare Suckow in Heusing. Zeitschr. f. d. Org. Phy. vol. ii. p. 24, &c. 



f In the larvae without an anus (Myrmecoleon, Vespas, Apis) the old skin remains 

 in the bag behind the stomach (compare . 105.), and is evacuated only after the pupa 

 state through the new-formed anus. 



