HARD WICKE'S SCIENCE- G OSSIF. 



107 



thorax. A rather long neck allows the head to be 

 retracted beneath the pronotum (first dorsal shield of 

 the thorax) or protruded beneath it. 



On the front of the head we observe the clypeus, 

 which occupies a large central tract, extending 

 almost completely across the widest part of the face. 

 It is divided above by a sharply bent suture from the 

 two epicranial plates, which form the top of the head 

 as well as a great part of its back and sides. The 

 labrum hangs like a flap from its lower edge. A 

 little above the articulation of the labrum the width 

 of the clypeus is suddenly reduced, as if a squarish 

 piece had been cut out of each lower corner. In the 

 re-entrant angle so formed the ginglymus, or anterior 

 •articulation of the mandible, is situated. 



The labrum is narrower than the clypeus, and of 

 squarish shape, the lower angles being rounded. 

 It hangs downwards, with a slight inclination back- 

 wards towards the mouth, whose front wall it forms. 

 On each side, about halfway between the lateral 

 margin and the middle line, the posterior surface of 

 the labrum is strengthened by a vertical chitinous 

 -slip set with large setae. Each of these plates passes 

 above into a ring, from the upper and outer part of 

 which a short lever passes upwards, and gives 

 attachment to a muscle {levator mcnti). 



The top and back of the head are defended by the 

 two epicranial plates, which meet along the middle 

 line, but diverge widely as they descend upon the 

 posterior surface, thus enclosing a large opening, the 

 •occipital foramen. Beyond the foramen, they pass 

 still further downwards, their inner edges receding in 

 .a sharp curve from the vertical line, and end below 

 in cavities for the articulation of the mandibular 

 •condyles.* 



The sides of the head are completed by the eyes 

 and the genre. The large compound eye is bounded 

 above by the epicranium ; in front by a narrow band 

 which connects the epicranium with the clypeus ; 

 behind, by the gena. The gena passes downwards 

 between the eye and the epicranial plate, then 

 •curves forwards beneath the eye, and just ap- 

 pears upon the front of the face, being loosely 

 connected at this point with the clypeus. Its lower 

 edge overlaps the base of the mandible, and encloses 

 the extensor mandibulse. 



The occipital foramen has the form of a heraldic 

 shield. Its lateral margin is strengthened by a rim 

 continuous with the tentorium, or internal skeleton 

 of the head. Below, the foramen is completed by 

 the upper edge of the tentorial plate, which nearly 

 ■coincides with the upper edge of the submentum 

 •(basal piece of the second pair of maxillae) ; a cleft, 

 however, divides the two, through which nerve- 



* One of the few points in which we have to differ from the 

 admirable description of the cockroach given in Huxley's 

 " Comparative Anatomy of Invertebrated Animals" relates' to 

 the articulation of the mandible, which is there said to be carried 

 iy the genit. 



commissures pass from the sub-oesophageal to the 

 first thoracic ganglion. Through the occipital fora- 

 men pass the oesophagus, the aorta, and the tracheal 

 tubes, for the supply of air to the head. 



The internal skeleton of the head consists of a 

 nearly transparent chitinous septum, named tentorium 

 Ijy Burmeister, which extends downwards and for- 

 wards from the lower border of the occipital foramen. 

 In front it gives off two long crura, or props, which 

 pass to the ginglymus, 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 readily understood from the figure 

 (fig- 59)' Its lower surface is strengthened by a 

 median keel which gives attachment to muscles. 

 The cesophagus passes upwards between its anterior 

 crura, the long flexor of the mandible lies on each side 

 of the central plate ; the supra-oesophageal ganglion 

 rests on the plate above, and the sub-cesophageal 

 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 Orthoptera, as well as in Neuroptera, and 

 Lepidoptera. Palmen (" Morphologic des Tracheen- 

 systems,"p. 103) thinks that it represents a pair of 

 stigmata or spiracles, which have thus become modi- 

 fied for muscular attachment, their respiratory function 

 being wholly lost. In Ephemera he finds that the 

 tentorium breaks across the middle when the skin is 

 changed, and each half is drawn out from the head, 

 like the chitinous lining of a tracheal tube. 



A pair of antennae spring from the front of the 

 head. In the male of the common cockroach they 

 are a little longer than the body ; in the female rather 

 shorter. From seventy-five to ninety joints are 

 usually found, and the three basal joints are larger 

 than the rest. Up to about the thirtieth, the joints 

 are about twice as wide as long ; from this point they 

 become more elongate. The joints are connected by 

 flexible membranes, and provided with stiff, forward- 

 directed bristles. The ordinary position of the 

 antennce is forwards and outwards. 



Three pairs of appendages are specially modified 

 as gnathites, or jaws. These are, in order from 

 before backwards, the mandibles, the maxillce, and 

 the labium, or second pair of maxillse. 



The mandibles are powerful, single-jointed jaws, 

 each of which is articulated by a convex "condyle" 

 to the lower end of the epicranial plate, and again by 

 a concave "ginglymus" to the; clypeus. The op- 

 posable inner edges are armed with strong tooth-like 

 processes of dense chitin, which interlock when the 

 mandibles close ; those towards the tip of the man- 

 dible are sharp, while others are blunt, as if for 

 crushing. Each mandible can be moved through 

 an angle of about 30°. The powerful flexor of the 



