SEGMENTATION OF THE HEAD 



On the basis of these contentions it will be observed that six 

 segments enter into the composition of the insect head, and this 

 conclusion is now widely accepted. Embryology affords three 

 fundamental criteria which provide the surest evidence with regard 

 to segmentation, viz., the existence of paired appendages, of 

 neuromeres and of primitive coelom sacs. Taking these three 

 criteria as the basis, the composition of the head will be discussed 

 in the light of recent investigations. A noteworthy addition to 

 knowledge of insect embryology is the work of Leuzinger, Wiesmann 

 and Lehman (1926) on the stick insect {Carausius morosus). It 

 is Wiesmann's contribution in this memoir that is of special 

 importance in the present connection. Besides confirming much 

 of what was previously known regarding the composition of the 

 insect head, this author brings forward new facts of considerable 

 theoretical significance. According to him the head of Carausius 

 is composed of an acron and six, or possibly seven, embryonic 

 segments, and his findings are expressed in tabular form below. 



Acron. 



1. Labral segment 



2. Pre-antennal segment 



3. Antennal segment 



4. Intercalary segment . 



5. Mandibular segment . 



6. 1st Maxillary segment 



7. 2nd Maxillary segment 



Without coelom sacs. 



Coelom sacs rudimentary, 



55 55 



, , present. 



,, rudimentary, 



,, present. 



Without appendages. 



Appendages ? 



Appendages rudimentary. 



Antennae. 



Appendages rudimentary. 



Mandibles. 



1st Maxillae. 



2nd Maxillae. 



Wiesmann's discovery of rudimentary coelom sacs in the labral 

 region (Fig. 2, B) was wholly unexpected, and this author has 

 suggested that a true somite, in front of the protocerebral segment, 

 is present. The labrum itself develops from paired lobes which he 

 believed may represent the appendages of this segment since they 

 are closely associated with the coelom sacs just mentioned. 

 Eastham (1930) states that in Pieris the labrum similarly arises 

 from a pair of lobes : these outgrowths fuse at their bases, pass 

 backwards with the stomodaeum and become connected with the 

 epipharynx. The pre-oral mesoderm becomes secondarily divided 

 into a principal anterior part, which passes into the hollow labral 



