1878.] 293 [Burgees. 



the fork and the outer lobe, and the peculiarity of the muscular con- 

 nections seem rather to favor the idea that the fork may represent an 

 independent organ. Perhaps the study of the early stages of the 

 Psocidae may decide this question. We will now examine the other 

 organs of the mouth. 



The clypeus (figs. 1 and 2, c) is always remarkably large and 

 vaulted, for the reception of a mass of muscles described below. 



The upper lip in Psocus is transverse, rather large, with the free 

 border well rounded. Two small hooks lie within this border, which, 

 moreover, is doubled over upon itself, about one-third the width of 

 the labium apart. They project backwards and inwards. In Atropos 

 these hooks are more developed, and serve to retain the two lobes of 

 the labium when placed against the labrum, while the labial palpi close 

 against the labrum outside of the hooks, thus completely covering 

 the mouth. Within, the labrum is furnished with a tuft of hairs 

 above, as shown in fig. 2. A pair of long muscles which open the 

 lip, are inserted one on either side of. the median line near the base 

 of the outer wall, and, running vertically to the cranium, are at- 

 tached a little above the clypeus (figs. 2 and 9, I. m.). 



The mandibles are very large and strong, outwardly presenting a 

 triangular aspect, the posterior side being concave (fig. 1, m.). 

 An inner view of the mandible (fig. 2) shows above a broad 

 molar surface transversely ribbed. This surface contracts below into 

 a sharp cutting edge, which curves backwards, forming a concavity 

 in which the tip of the maxillary lobe lies. The cutting edge is 

 slightly denticulated, as seen in fig. 8. The mandibles are moved by 

 two enormous flexor muscles (figs. 2 and 8, /. m.), which fill the 

 greater part of the cranium. They are made of a number of bun- 

 dles radiating as usual from a chitinized tendon, which is inserted 

 on the posterior upper edge of the mandible. The extensors are 

 weak muscular bundles inserted on the middle of the upper outer 

 edge of the mandible, and attached within the cheeks just below 

 the eye (fig. 8, e., m.). 



The maxilla? and mandibles occupy the lower half of the large 

 oral cavity, which opens above into a thick-walled oesophagus. Be- 

 low the opening of the oesophagus lies a bone, which may be'fanci- 

 fully likened to a lady's bonnet upside down (fig. 2, ce. &., fig. 10) ; 

 the high front lies along the oral cavity at about half way up; two 

 narrow extensions, representing the bonnet strings, run forwards and 

 upwards, embracing the oesophagus (fig. 9). The great bundles of 



