OR OSSKOrs SYSTEM. 37 



compose the foot which commonly terminates with simple 

 crooked ungues or opposeable chili. 



The segments of the abdomen, commonly divided into 

 an upper and a lower piece connected together at the 

 sides by an unconsolidated portion of the integument, 

 encompass the digestive and the generative organs, which 

 for the most part terminate in the last segment. Each 

 of the abdominal and thoracic segments is perforated on 

 each side by a small spiracle or stigma which leads into 

 the respiratory tracheae ramified through every part of the 

 body. The wings and the legs are the parts most subject 

 to variations of form and magnitude, and their variations 

 are accompanied with corresponding differences in the 

 relative development of the thoracic segments which lodge 

 the muscles and nerves of these locomotive organs. In 

 insects where the exterior or first pair of wings are 

 little used in progression, as in most of the coleoptera, 

 the mesothorax is small, and the other two segments, 

 the anterior and posterior, are large ; but in dipterous 

 insects, where the anterior or first pair of wings are 

 alone developed, the mesothorax has assumed a pro- 

 portionally great development, and the pro- and meta- 

 thorax are very small. In the hymenoptera, the meso- 

 thorax is large, in the hemiptera and orthoptera, the 

 anterior segment is most developed, and in the neurop- 

 terous insects, the meso- and the meta-thorax are large 

 compared with the anterior thoracic segment. The meso- 

 thorax is commonly the most important segment of this 

 region of the body, and that which best shows its com- 

 ponent elementary pieces. We observe four elements 

 on its tergum or dorsal surface, arranged in a single lon- 

 gitudinal series, the anterior of which is the prse-scutum ; 

 then follow the scutum, the scutellum, and the post- 

 scutellum. On the anterior aspect, or pectus, of the same 

 segment^ are seen the sternum single on the median plain, 

 and arranged in pairs on its sides, the paraptera, the epis- 

 ternum, and the epimera. These tegumentary parts, form- 

 ing the skeleton of insects, are thrown off about five 

 times during the larva state, and once from the chry- 

 salis before the animals assume their perfect adult 

 form. 



