54O HEMIPTERA CHAP. 



corium is differentiated from the rest of the surface, and is termed 

 the " cuneus." In Plataspides, one of the divisions in which 

 the alar organs are entirely covered by the scutellum, they are 

 modified in a very remarkable manner. In the Homoptera the 

 divisions named above do not exist, and the wings in repose are 

 placed in a different position, as stated in our definition of the 

 Order. It is said to be very difficult to homologise the wing- 

 nervures of Hemiptera, and nothing appears to be known as to 

 the mode of their development. 



The alar organs in Hemiptera exhibit a very frequent form 

 of variation within the limits of the same species ; this has not 

 yet been elucidated. 1 In some cases in the Heteroptera nearly 

 all the individuals of a generation may have the wings aborted ; 

 sometimes this occurs as a local variation. In Aphidae the 

 occurrence of winged and wingless individuals is very common, 

 and has even become an important factor in their extraordinary 

 life cycles. (See Chermes, etc., subsequently.) 



Internal anatomy. The alimentary canal presents consider- 

 able diversity and some remarkable features. There is a slender 

 tube-like oesophagus and a large crop. It is difficult to assign 

 any of the parts posterior to this to the divisions usual in 

 other Insects, and it is said that the distinction of parts histo- 

 logically is as vague as it is anatomically. In the Heteroptera 

 the Malpighian tubes open into two (or one) vesicular dilatations 

 seated immediately in front of the short rectum : between this 

 point and the crop there may be a very elongate, slender portion 

 with one or more dilatations, these parts apparently replacing 

 the true or chylific stomach. There is no gizzard. In the 

 Homoptera the relations of the divisions of the alimentary canal 

 are even more puzzling ; the canal is elongated and forms coils, 

 and these are connected with tissues and tunics so as to make 

 their dissection extremely difficult. List says that there are 

 great differences in the alimentary canal among the members 

 of the one family Coccidae. There are usually four Malpighian 

 tubes, but in Coccidae there is only one pair, and in Aphidae 

 none. The excretory cells of these tubes are in Hemiptera of 

 remarkably large size. There is a large development of salivary 

 glands, at least two pairs existing. There can be little doubt 

 that some of their products are used for purposes of injection, as 



1 On tliis subject, see Reuter, Ann. Soc. ent. France (5) v. 1875, p. 225. 



