TRACHEATA. 



407 



cases by the division of the mesoblastic bands into 

 separate somites. The most anterior line marks off a pra-oral 

 segment, which soon sends out two lateral wings the proccphalic 



The remaining segments are at first fairly uniform. 

 Their number does not, however, appear to be very constant. 

 So far as is known they never exceed seventeen, and this 

 number is probably the typical one (figs. 186 and 187). 



In Diptera the number appears to be usually fifteen though it may be 

 only fourteen. In Lepidoptera and in Apis there appear to be sixteen 

 segments. These and other variations affect only the number of the segments 

 which form the abdomen of the adult. 



The appendages arise as paired pouch - 

 like outgrowths of the epiblast and meso- 

 blast ; and their number and the order of 

 their appearance are subject to considerable 

 variation, the meaning of which is not yet 

 clear. As a rule they arise subsequently to 

 the segmentation of the parts of the body 

 to which they belong. There is always 

 formed one pair of appendages which spring 

 from the lateral lobes of the procephalic 

 region, or from the boundary line between 

 these and the median ventral part of this 

 region. These appendages are the antennae. 

 They have in the embryo a distinctly ven- 

 tral position as compared to that which 

 they have in the adult. 



In the median ventral part of the pro- 

 cephalic region there arises the labrum (fig. 187, /j). It is formed 

 by the coalescence of a pair of prominences very similar to true 

 appendages, though it is probable that they have not this 

 value 1 . 



1 If these structures are equivalent to appendages, they may correspond to one of 

 the pairs of antennae of Crustacea. From a figure by Fritz Miillcr of the larva of 

 Cnlotermes (Jcnaischt Ztit. Vol. XI. pi. n, fig. n) it would appear that they lie in 

 front of the true antenna-, and would therefore on the above hypothesis correspond to 

 the first pair of antenna- of Crustacea. Butschli (No. 405) describes in the Bee a pair 

 of prominences immediately in front of the mandibles which eventually unite to form 

 a kind of undcrlip; they in some ways resemble true appendages. 



FIG. 186. EMBRYO 

 OK HYDROPHILUS PI- 

 CEUS VIEWED FROM THE 

 VENTRAL SURFACE. 

 (After Kowalevsky.) 



pc. I. procephalic lobe. 



