215 



TcleaSj the Clialciclites Pentameri from the Chalcidites Tetraraeri, the 

 positive pole being represented by the first group of each pair, and 

 the negative pole by the second. In the Hemiptera the Reduviites 

 and Notonectites are superior to the Pentatoraites and Aphites ; and 

 in the Diptera the higher pole appears next the Asilites, the lower 

 pole next the Muscites. The Iiej)idoptera have comparatively but 

 little variation. 



The number of primary segments along the longitudinal axis of the 

 class Insecta is invariably thirteen, and in this character they differ es- 

 sentially from the Myriapoda, the number of whose segments are pro- 

 gi-essively increased from six to fifty and upwards, and during the 

 whole of their life segment after segment is added in continual deve- 

 lopment toward the lower pole. The larvae of insects are also in- 

 creased in the same direction, but in the last state of life they are 

 exalted by an opposite tendency, and therefore the number of segments 

 is limited, and when they begin to assume the perfect form, the mat- 

 ter elaborated from food is carried toward the head and the adjoining 

 segments, and their appendages. The economy of Hymenoptera, 

 and especially of the Aculeata and Pupivora, requires the abdomen to 

 perform essential functions, and accordingly its development, with 

 that of the head and thorax, causes the intermediate part to be much 

 diminished and to form a petiole. The Amorpha of Newman, com- 

 prising the Lepidoptera and Diptera, exhibit most of this passage 

 between the poles ; next come the Necromorpha, or the Hymenoptera 

 and the Coleoptera ; and lastly the Isomorpha, or the Orthoptera and 

 the Hemiptera, in whose metamorphose, and in that of some of the 

 Anisomorpha or Neuroptera, the region of the higher pole has the 

 least tendency to development at the expense of that of the lower. 

 The thirteen segments occur in the following order fi'om the higher 

 pole to the lower : — 1, Caput ; 2, Prothorax ; 3, Mesothorax ; 4, Me- 

 tathorax ; 5, Propodeon ; 6, Podeon ; 7, Metapodeon ; 8, Octoon ; 9, 

 Ennaton ; 10, Decaton ; 11, Protelum ; 12, Paratelum ; 13, Telum; 

 and the perfect composition of one segment is invariably attended 

 with the imperfect structure of another. When a segment ( such as 

 the mesothorax of most insects, and the octoon of many Hymenop- 

 tera) is much developed, it ceases to have freedom of motion, but 

 governs the movements and offices of its appendages or of other seg- 

 ments. 



Francis Walker. 



