NO. 3 EMBRYOLOGY OF FLEAS KESSEL 57 



There is some variation as to the definitive position of the gonads in 

 the flea larva. During development, the body of the sex organ is 

 observed to lie in parts of three different segments. These are the 

 fourth, fifth, and sixth abdominal segments. The definitive larval 

 gonads lie in one of these somites. Lass (1905) says that the larval 

 ovaries are located in the sixth abdominal segment whereas the testes 

 are situated further anterior. If his differentiation is correct, it may 

 be assumed that the gonad anlagen of the female contract to form 

 the larval ovaries which are restricted to the sixth abdominal seg- 

 ment. Likewise, the anlagen cf the testes contract to occupy positions 

 anterior to their female counterparts. These are in the region of the 

 fourth abdominal segment. 



TRACHEAL SYSTEM 



The tracheae of fleas, like those of other insects, arise as paired 

 invaginations of the ectoderm. These are segmental in arrangement. 

 They first become evident about the time the coelomic sacs make their 

 appearance and while the neural groove is still open (pi. 2, fig. 28). 

 They are situated near the lateral margins of the germ band. The 

 mouths of the invaginations ultimately become the spiracles, while 

 the invaginations themselves deepen, branch, and anastomose to form 

 the complex respiratory system of the larva. A longitudinal section 

 along one of the tracheae is shown in figure 30 (pi. 2) . The definitive 

 number of spiracles found in the fully developed flea embryo is 10 

 pairs. These are located on the prothorax, the metathorax, and the 

 first eight abdominal segments. All of them arise in their definitive 

 position except the first, which originates on the mesothorax and 

 migrates to its larval position during embryonic development. Its 

 position near the posterior margin of the prothoracic segment is 

 indicative of its mesothoracic origin. Evanescent tracheal invagina- 

 tions, such as have been observed in Leptinotarsa (Wheeler, 1889) 

 and Calendra (Wray, 1937), originating in the prothoracic segment, 

 were not found in the flea embryo. Likewise, no transitory tracheal 

 pits corresponding to those described by Nelson (1915) as occurring 

 on the labial segment of the embryo of Apis were discovered in this 

 study. Tracheal invaginations of a rudimentary nature, arising on 

 abdominal segments posterior to the eighth like those reported in 

 Lepisma (Heymons, 1897), Leptinotarsa (Wheeler, 1889), and Ca- 

 lendra (Wray, 1937), also appear to be lacking in the development of 

 fleas. 



