DIGESTIVE CANAL OF AETHROPODA. 273 



In some cases tlie mid-gut ends blindly and Las no connection 

 with a hind-gut. This happens in the larva of Bees and Wasps, of 

 the Iclmeumonida, and of many Diptera, etc. 



The hind-gat in the Insecta generally forms the shortest portion 

 of the straight alimentary canal. It is often divided into two por- 

 tions, the second of which is widened out ("rectum") (Fig. 142, A B r). 

 In the Coleoptera (as in Dytiscus) its narrower and anterior por- 

 tion appears to be of considerable length, as in many Orthoptera, 

 where a larger number of divisions, varying in width, can be made 

 out ; it is longest in the Cicadas : in all of these forms it is disposed 

 in coils. As in many the Malpighian vessels, which belong to the 

 hind-gut, open very far forwards, it appears as if part of the last 

 division of the hind-gut had passed into the mid-gut, although, 

 indeed, this rather seems to indicate that the mid-gut has been 

 reduced in length. 



The widened terminal portion of this part of the enteric canal is 

 distinguished in a large number of Insects by papilliform ridges, 

 which project into it : in these we find many tracheal branches. This 

 portion in many aquatic larvas of the Libellulidas has numerous 

 lamellae arranged in longitudinal series, and provided with closely- 

 packed tracheal ramifications. The lamellas act as organs of respira- 

 tion during the in-and-out flow of water, which is caused by the 

 opening and closing of the anus (cf. Fig. 150, B G). There are 

 many intermediate forms between these tracheal gills, and the 

 papilliform processes of the hind-gut (in the larvas of the Phry- 

 ganeidas), so that these organs can be seen to be homologous. 



Plateau, F., Eech. sur les Pheuomenes de la digest, chez lcs Inseetes. Mem. 

 Acad. Belg. XLI. 



Organs appended to the Digestive Canal. 



1) Appendages of the Fore-Gut. 



§ 212. 



Glandular organs are differentiated in the various portions of 

 the alimentary canal of the Arthropoda. The salivary glands, 

 which open into the fore-gut, are but slightly developed in the 

 Crustacea. Groups of unicellular glands have been made out in 

 some divisions. On the other hand, we find them more widely 

 distributed in the Tracheata, where they may have various functions ; 

 but as yet we have but very little exact information as to how they 

 open. The great point which is undetermined is, how far they open 

 into the mouth itself, or are connected with the fore-gut. But the 

 glands which open into the gnathites of Peripatus (§ 193) must be 

 mentioned here. Among the Arachnida, the Scorpions have two 



