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ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



eyes (E) . In many insects there are three simple eyes, distinguished 

 as ocelli (O), placed on the top of the head or on the upper part of 

 the face. From the lower part of the face there hangs before the 

 jaws a free transverse lobe, the Idbrum (Lm). Behind the labrum 

 are suspended from the lower lateral margins of the cranium three 

 pairs of feeding appendages, or gnathopods {Md, IMx, 2Mx), which 

 are' legs transformed into organs for manipulating and chewing the 

 food. The cavity of the head communicates with that of the body 

 by an opening in the posterior wall of the cranium, usually of large 

 size (figs. 9 B, 13, For), commonly called the occipital foramen, 

 though by analogy it corresponds with the fora^jien magnum of a 

 vertebrate skull. 



iMx 2Mx(Lb) 



Figure 3. — Diagrams showing ttie general structure of an insect's liead and tlie 

 relations of the mouth parts to the cranium 



A, lateral view. B, transverse section, a, dorsal articulation of basis of a 

 gnathal appendage with lower edge of cranium ; Ant, antenna ; at, anterior 

 tentorial pit ; 6, ventral end of basis ; Bnd, basenditos. or mesal lobes of 

 appendage basis ; Cv, cervix, or neck ; cvpl, cervical scleritcs ; D, dorsum ; 

 E, compound eye ; es, epistomal suture ; Hphy, hypopharynx ; LB, basis of the 

 appendage ; Lh, labium ; Lm, labrum ; Md, mandible ; IMx, first maxilla ; 

 ZMx, second maxilla ; O, ocelli ; occ, occipital condyle ; oca, occipital suture ; 

 P, P, pleural areas ; pt, posterior tentorial pit ; 8gR, subgenal ridge ; sgs, 

 subgenal suture; Tlpd, telopodite (palpus) of appendage; Y, venter. 



If we analyze a cross-section through the posterior part of the 

 head (fig. 3 B) in terms of the structure of a body segment cut trans- 

 versely, we arrive at the following results. The cranial wall above 

 the dorsal articulations of the appendages (a, a) is the dorsum (Z>), 

 and its sclerotization represents the tergum of a body segment; the 

 ventral wall (F) between the lower ends of the appendage bases 

 (6, h), including the hypopharynx {Hphy), is the venter, or region 

 of a body segment that contains the sternum ; the ventrolateral areas 

 (P, P), in which the appendages are broadly implanted correspond 

 with the so-called pleural areas of a body segment; and finally, the 

 appendages themselves represent a pair of legs, each with an enlarged 



