288 



COMPAli ATIVE ANATOMY. 



These organs appear to be distributed in a metameric fashion even 

 in the cephalic segments, for the rudiments of tracheae formed by the 

 ectoderm have been observed in many Insects in these metameres 

 during their development. None of these rudiments are retained in 

 any living Tracheate, owing to the development of the head. In the 

 succeeding metameres, also, the number of tracheae may be diminished, 

 for in some cases at any rate the tracheal trunks have been observed 

 to atrophy. 



In the Myriapoda the tracheae are ordinarily similar in character 

 throughout the whole body, however different they may be in the 

 various orders. The stigmata, which are either placed on the 

 ventral surface, or more to the sides, and in some indeed on the 

 dorsal surface (Scutigera), lead into tracheal trunks, which are dis- 

 tributed in correspondence with the number of metameres. They 

 are most simply arranged in Julus. A tuft of tracheae passes from 

 each stigma to the viscera, without branching at all. In Glomeris, 

 however, the tracheaa do branch; and in the Chilopoda they form 

 longitudinal and transverse anastomoses, so that they get to be 

 arranged in very much the same way as in many Insects. 



Among the Insecta some Aptera appear to have lost their trachea?. 

 They are almost altogether wanting in the Collembola, two pro- 

 thoracic trachea? only having been observed in Smynthurus. Among 

 the Thysamrra, Campodea has as many as three pairs (Fig. 150), 



belonging to the mesothoracic, metathoracic, and 

 first abdominal segment respectively. The absence 

 of anastomoses shows that they are of a lower 

 grade of development than that found perma- 

 nently in other insects. There are generally 10 

 pairs of stigmata. This is the highest number 

 known in the imago stage of the Pterygota, but 

 in some larvas there are 11 stigmata, the first 

 abdominal metamero being sometimes provided 

 with a stigma, although as a rule it is altogether 

 absent. There are never any stigmata in the last 

 two metameres. The number of these stigmata, 

 and of the tracheal trunks which arise from them, 

 is not, however, always complete. The number 

 varies greatly, pairs of stigmata undergoing de- 

 generation at one point or another, so that only 

 2 or 3 of them remain open. In the imago they 

 generally lie in the softer membrane, which con- 

 nects the segments of the body, and they are 

 sometimes placed so much on the dorsal surface 

 of the abdomen that they are covered by the 

 wings (Coleoptera). The number and arrange- 

 ment of the trachea) in the imago stage is not the same as in the 

 pupa3 or larvae. The differences in the external conditions which 

 obtain in the two stages define the arrangements of this system of 

 respiratory tubes. The development of transverse and longitudinal 



Fio-. 150. Anterior 

 half of Campodea 

 fragilis. s Stig- 

 mata (after Palmen). 



