THE STRUCTURE OF THE HEAD OF THE BLOWFLY LARVA. 121 



rudiments of these structures are developed iu the embryonic stage. 

 Comparative morphology teaches us that the mandibles and 

 maxilla? are modified limbs, their manner of development being the 

 same as that of normal limbs. 



Much discussion has arisen with regard to the segmentation of 

 the head in insects. Some regard it as consisting of five, others of 

 six or more segments. In former days my mind was much exercised 

 with this and similar problems, but embryology has shown me the 

 futility of such discussions. There is a close analogy between this 

 controversy and one, now almost forgotten, on the' vertebral nature 

 of the skull. Oken and Gcethe initiated the comparison of the pre- 

 and post-oral structures in insects and crustaceans as well as the 

 vertebral theory of the skull. Huxley long ago disposed of the 

 latter question by showing that no initial' segmentation precedes 

 the formation of the skull as it does that of the vertebral column. 

 The evidence of embryology is no less decisive, in insects and 

 arthropods generally ; as no segmentation occurs in the pre-oral 

 region, and the head consists of an unsegmented pre-oral cap, 

 developed from the cephalic fold, of two lateral procephalic lobes, 

 and of three post-oral segments, with their three pairs of lateral 

 appendages. From the latter the mandibles, maxillae, and labium 

 are developed. These are, therefore, in serial homology with the 

 thoracic and abdominal feet. The antennas, on the other hand, 

 are developed from the non-segmented pre-oral region, and, like 

 the eyes, have no homologies with limbs. A comparison between 

 these structures and the post-oral appendages has no more basis in 

 their developmental history than a comparison of the trabaeculee 

 cranii with the ribs, or of the sense capsules of a vertebrate with 

 its limbs. 



Almost as soon as the embryonic structures above described are 

 formed in the egg of the fly, the anterior pair of maxilla? exhibit a 

 tendency to enlarge and become parallel to each other, whilst the 

 rudiments of the mandibles and the segment which supports them, as 

 well as those of the second pair of rnaxilla?, and the segment be- 

 longing to them, undergo retrograde changes. 



These facts were observed and recorded long ago by Dr. Weis- 

 mann.* Ultimately, as Weismann correctly states, the mandibles 

 and the mandibular segment disappear. It is not so, however, with 

 the posterior pair of maxillaa; these coalesce and form a small three- 



* " Z. F. W. Z.," Bd. 13. 



