COMMON COCKROACH. 141 



front, which is articulated by a distinct joint to the stipes ; and (5) a lacinia. This 

 lacinia is attached to the stipes by an imperfect joint; it ends with two stout 

 conical teeth, at the base of which there rises on the internal edge a finger-shaped 

 process, terminated by three or four recurved blunt teeth. Its inner edge is beset 

 with stout hairs. The labium closes in the mouth posteriorly. It consists (i) of a 

 large basal sub-mentum, to the fore-edge of which is moveably articulated (2) a 

 mentum ; (3) of two three-jointed palpi attached each to a basal prominence, repre- 

 senting a palpiger borne on the external angle of the mentum ; and (4) a ligula. 

 The ligula is divided almost completely by a median cleft, at the base of which is a 

 small triangular piece. Each half of the ligula bears two processes articulated to 

 it : an outer, the paraglossa or lamina externa of Gerstacker ( = galea ?), and an 

 inner, the smaller of the two, the lamina interna of the same author (= lacinia?). 

 A labium of this character is to be regarded as primitive. It is found in many 

 Orthoptera, in Termes, Perla, Aeschna, and the incomplete stages of Ephemeridae, 

 and shows clearly the origin of the labium from a pair of limbs fused medianly. 

 The antennae are processes of the pro-cephalic lobes : while the mandibles, 

 maxillae, and labium belong to three fused segments distinct in the embryo. The 

 head of the insect is consequently often regarded as formed of four segments, one 

 prae-oral and three post-oral 



The labrum (supra] is formed in some Insecta by the fusion of two processes. 

 They are, however, hardly to be regarded as limb-rudiments. The antennae are 

 not modified limbs like the remaining appendages of the head, and are compared 

 by Balfour with the paired processes of the prostomium in the Chaetopod Polychaeta. 

 The embryonic rudiments of the labium are extremely large, and in some Insecta 

 are turned backwards parallel to the thoracic limbs. 



The epipharynx, which lies on the internal surface of the labrum, is not trace- 

 able in Periplaneta as a distinct process. The hypopharynx (= lingua), on the 

 contrary, is very large. It lies on the inner or oral surface of the labium, and the 

 salivary duct opens towards the base of its posterior surface. 



The thorax is composed of three limb-bearing segments pro-, meso-, and 

 meta-thorax. A distinct neck 1 intervenes between the thorax and the head. The 

 pro-thorax has a large tergal (dorsal) plate, the pro-notum, which overlaps the head 

 in front and the meso-thorax behind. Its sternal element is small, as are the two 

 lateral elements, epi-merum, and episternum, which lie in front of the articulation 

 of the limb. These parts are larger in the two segments behind. The tergal 

 elements (meso- and meta-notum) of the meso- and meta-thorax are sub-equal in 

 the Cockroach. In most Orthoptera, some Neuroptera, the meta- is larger than 

 the meso-thorax, a condition the reverse of what obtains in most Insecta. The 

 limbs increase in length progressively from before backwards. Each limb is made 

 up of ten joints a large coxa articulating with the thoracic ring, a small trochanter 

 followed by a femur, a tibia, and a tarsus of six joints. The last tarsal joint is 

 minute, and furnished with tivo claws, as is usual in Insecta that possess claws. 

 The meso- and meta-notum carry wings. Those of the first pair in the male are 



1 In the neck there are certain chitinous pieces, or cervical selerites, one dorsal and median, 

 with a longitudinal depression, two ventral and transverse, and two lateral. The first and second 

 elements are inconspicuous, the third large. They are placed obliquely. Their significance is doubt- 

 ful. See Huxley, Anatomy of Invertebrated Animals, p. 403. 



