2 Psyche [February 



with the mandible of mandibulate insects. The lateral edges of the 

 clypeus are often turned down and produced into free plates (c) 

 projecting forward and fitting in between the labrum and the 

 maxillary plates (mx p). The separation of the clypeus and the 

 labrum is often difficult to recognize in an external view, but 

 internally the anterior part of the pharynx is fastened to the 

 anterior part of the clypeal region by two chitinous struts or 

 pillars; beyond this point the walls of the pharynx separate into 

 the epipharynx and hypophary nx ; the pharyngeal muscles are 

 attached to the wall of the clypeus. The division between the 

 labrum and the epipharynx is often very obscure, and it may be 

 more convenient to use the compound word "labrum-epipharynx," 

 as is done in the Diptera. The latero- ventral coverings of the 

 mouth are formed by two plates {mx p), the anterior edges meet- 

 ing along the ventro-medial line, thus closing the mouth below 

 and beyond the hypophary nx and syringe. The maxillary setae 

 pass through the membrane (hypodermis) at the base of the inner 

 side, and the protractor muscles run forward and attach them- 

 selves to the walls, of these plates. We homologize these plates 

 as the maxillae of mandibulate insects, and the setae as maxillary 

 setae. Judging by Prof. Smith's study of the development of the 

 palpifers in many insects, it is probable that the maxillary setae 

 in this order and in Thysanoptera are also developments of the 

 palpifers. In the Homoptera the gular region is reduced to a 

 minimum, in many cases being but a thin membrane connecting 

 the labium with the occipital region; this causes the deflection of 

 the head. The occipital foramen is very large. Posterior to the 

 antennae the head-capsule is often developed into a "frontal" or 

 antennal ridge (fr) ; this plays but a small part in the Homoptera, 

 but in the Heteroptera it is large, and greatly modifies the shape 

 of the head. 



In many of the Hemiptera (i. e., Siphanta, Fig. 5) the mandi- 

 bular setae {man. s.) are broad and bent a little beyond the base, 

 protractor and retractor muscles proceeding to the head-capsule. 

 In such forms their homology to mandibles is clear, but in other 

 Hemiptera, especially among the Heteroptera, the tendon of the 

 retractor muscle is greatly developed, and appears as the basal 

 part of the setae proceeding to the back of the head, the bent 

 true basal portion being greatly reduced and specialized; in such 



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