50 TEXT-BOOK OF ENTOMOLOGY 



Palme'n regards the tentorium as representing a pair of tracheae (with the 

 cephalic spiracles) which have become modified for supports or for muscular 

 attachment, since he finds that in Ephemera the tentorium breaks across the 

 middle during exuviation, each half being drawn out of the head like the chiti- 

 nous lining of a tracheal tube. This view is supported by Wheeler, who has 

 shown that the tentorium of Doryphora originates from five pairs of invagina- 

 tions of the longitudinal commissures, and which are anterior to those of the 

 second maxillary segment. " These invaginations grow inwards as slender 

 tubes, which anastomose in some places. Their lumina are ultimately filled 

 with chitin." (Jour. Morph., iii, p. 368.) 



This view has also been held by Carriere and Cholodkowsky, but Heymons 

 concludes from his embryological studies on Forficula and Blattidaa (1895) that 

 it is unfounded. That this is probably the case is proved by the fact that the 

 apodemes of the thoracic region are evidently not modified ti'acheaa, since the 

 stigmata and tracheae are present. 



Number of segments in the head. While it is taken for granted 

 by many entomologists that the head of insects represents a single 

 segment, despite the circumstance that it bears four pairs of appen- 

 dages, the more careful, philosophical observers have recognized the 

 fact that it is composed of more than a single segment. Burmeister 

 recognized only two segments in the head ; Carus and Audouin recog- 

 nized three ; Macleay and Newman four ; Straus-Durckheim even so 

 many as seven. Huxley supposed that there are five segments bear- 

 ing appendages, remarking, " if the eyes be taken to represent the 

 appendages of another somite, the insect head will contain six 

 somites." (Manual of Anat. Invert. Animals, p. 398.) 



These discordant views were based on the examination of the head 

 in adult insects ; but if we confine ourselves to the imago alone, it is 

 impossible to arrive at a solution of the problem. 



Newport took a step in the right direction when he wrote : " It 

 is only by comparing the distinctly indicated parts of the head in 

 the perfect insect with similar ones in the larva that we can hope to 

 ascertain the exact number of segments of which it is composed." 

 He then states that in the head of Hydrous iiiceiis are the remains 

 of four segments, though still in the next paragraph, when speaking 

 of the head as a whole, he considers it as the first segment, " while," 

 he adds, " the aggregation of segments of which it is composed we 

 shall designate individually subsegments." 



That the head of insects is composed of four segments was shown 

 on embryological grounds by the writer (1871) and afterwards by 

 Graber (1879). The antennae and mouth-parts are outgrowths bud- 

 ding out from the four primitive segments of the head; the antennae 

 grow out from the under side of the procephalic lobes, and these 

 should therefore receive the name of antennal lobes. In like manner 



