GENERAL EXTEENAL STEUCTUBE OF IISTSECTS. 13 



Observers differ concerning the fate of the seventh segment, but it 

 is most probable that a part of it fuses with the sixth segment, thus 

 taking part in the formation of the head, and that a part of it forms 

 the neck or some of the neck plates of the adult. 



The appendages of these first seven segments form the antennse 

 and mouth parts, except one or two pairs that disapjaear early in 

 embryonic life. It is not certain that the first segment ever possesses 

 appendages, but from it arise the large compound eyes and appar- 

 ently also the upper lip, or labrum (Lm). The appendages of the 

 second segment form the feelers, or antennae (lAnt) of the adult, 

 those of the third (^Ant) disappear in insects, but they correspond 

 with the second antennae of shrimps and lobsters. The appendages 

 of the fourth segment form the mandibles (Md). Those of the 

 fifth segment (Slin), when present, fuse with a median tonguelike 

 lobe (Lin) of the following segment, and the three constitute the 

 hypopharynx, or lingua of the adult. The next pair {IMx) form the 

 maxillae, while the last {£Mx), or those of 'the seventh segment, 

 coalesce with each other and constitute the adult labium, or lower lip. 



The bodies of the head metameres fuse so completely that it is 

 impossible to say positively what parts of the adult head are formed 

 from each. The last, as already stated, possibly takes part in the 

 formation of both the head and the neck. Some embryologists at- 

 tribute the plates which usually occur in this region to the last em- 

 bryonic head segment, while others believe they come from the next 

 segment following. Sometimes these plates are so well developed 

 that they appear to constitute a separate segment in the adult, and 

 this has been called the mAcrothorax. If this name, however, is 

 given to the embryonic segment from which these plates are said to 

 be derived, it must be remembered that it is not " thoracic " at all 

 and belongs partly to the head. The name cervicum has been ap- 

 plied to the neck region with greater appropriateness since it does 

 not imply any doubtful affiliation with adjoining regions. What 

 we really need, however, is not so much a name as more information 

 concerning the development of the rear part of the head and the 

 neck plates in different insects. 



The next three segments remain distinct throughout life in nearly 

 all insects, but, since they bear the legs and the wings, they become 

 highly specialized and together constitute the thorax. The indi- 

 vidual segments are designated the prothorax, the mesothorax, and 

 the metathorax. The legs are formed from the embryonic ap- 

 pendages (fig. 2, IL, ^Z, 3L) of these segments, but the wings are 

 secondary outgrowths from the mesothorax and metathorax and 

 are, hence, not appendages in the strict embryological sense. 



The remaining segments, nearly always 10 in number, constitute 

 the abdomen. The appendages of these segments, except possibly 



