folsom: mouth-paets of anueida maritima. 141 



dtlnnen Chitiustab, welcher oben im Kopfe, hinter den Augen, endet 

 uud da am Chitin des Kopfes festsitzt." 



Thus, the tentorium of Machilis is constructed upon the same plan as 

 that of Anurida, although the median plate is halved longitudinally. 

 The dorsal and posterior arms in Anurida are clearly represented in 

 Machilis, and the latter pair tends to become reduced in size, — an 

 approach to the Orthopteran condition. 



The tentorium in Orthoptera is readily comparable with that of 

 Machilis. In Periplaneta, according to Miall and Denny ('86, p. 39), 

 "In front it gives off two long crura, or props, which pass to the gin- 

 glymus, and are reflected thence upon the inner surface of the clypeus, 

 ascending as high as the antennary socket, round which they form a 

 kind of rim. Each crus is twisted, so that the front surface becomes 

 first internal and then posterior as it passes towards the clypeus. The 

 form of the tentorium is in other respects i-eadily understood from the 

 figure (Figure 17). Its lower surface is strengthened by a median keel 

 which gives attachment to muscles. The oesophagus passes upwards 

 between the anterior crura, the long flexor of the mandible lies on each 

 side of the central plate ; the supraoesophageal ganglion rests on the 

 plate above, and the suboesophageal ganglion lies below it, the nerve- 

 cords which unite the two passing through the circular aperture. A 

 similar internal chitinous skeleton occurs in the heads of other Orthop- 

 tera, as well as in ISTeuroptera and Lepidoptera." 



In Anabrus (Packard, '98, p. 49, Figure 33) the tentorium is essen- 

 tially the same, with a central plate, and paired dorsal and posterior 

 arms. The only important differences between Orthoptera and Collem- 

 bola in respect to the tentorium are (1) that the oesophageal commissures 

 pass through it in the former group instead of around it; (2) that in 

 Orthoptera the posterior arms are reduced in length, and (3) that the 

 tentorium becomes more stoutly chitinized. On the other hand, the 

 tentorium of Orthoptera, in its general form and topographical relations, 

 agrees closely with the same structure in Collembola and Thysanura. 



Palmen ('77) derived the tentorium from a pair of cephalic trachea? in 

 Ephemera, but upon unsatisfactory grounds. In Collembola tracheae 

 are absent; moreover, as Packard ('98, p. 50) notes, "the apodemes of 

 the thoracic region are evidently not modified tracheae, since the stigmata 

 and trachese are present." 



The views of Carriere ('90) and Cholodkowsky ('91), agreeing with 

 the opinion of Palmen, have been controverted by Hej-mons ('95*', 

 pp. 50-51). 



