Most of the members of this series are remarkable on account of the great concentration of the nerve-centres. This is extreme in _Rhizotrogus_, where there are only two great ganglia, viz. the supra-oesophageal and a great ganglion situate in the thorax, and consisting of the conjoined infra-oesophageal, thoracic, and abdominal ganglia. According to Brandt[80] there are several distinct forms of concentration in the series; the Lucanidae only participate in it to the extent that the perfect Insects exhibit fewer ganglia than the larvae; the latter possess two cephalic, three thoracic, and eight abdominal ganglia, while the perfect Insect has the abdominal ganglia reduced in number to six, and {192}they are placed partially in the thorax. The diminution in number takes place in this case by the amalgamation of the first two abdominal with the last thoracic ganglia. [Illustration: Fig. 86—View of one side of meso- and metathorax of a Passalid larva from Borneo showing the stridulating organs. _a_, _b_, Portions of the metathorax; _c_, coxa of 2nd leg; _d_, striate or stridulating area thereon; _e_, basal part of femur of middle leg; _f_, hairs with chitinous process at base of each; _g_, the diminutive 3rd leg modified for scratching the striated area. × 20.] FAM. 1. PASSALIDAE.—_Labrum large, mobile; mentum deeply cut out in the middle for the accommodation of the ligula; the lamellae of the antenna brought together by the curling up of the antenna. The elytra entirely cover the dorsal surface of the abdomen._ There are four or five hundred species of this family known; they are usually shining-black, unattractive beetles, of large size, and are abundant in the decaying wood of tropical forests. They are quite absent from Europe, and there is only one species in the United States of North America. The larvae are very interesting, from the fact that they appear to have only four legs. This arises from the posterior pair being present only as very short processes, the function of which is to scrape striated areas on the preceding pair of legs and so produce sound. In the species figured (Fig. 86) this short leg is a paw-like structure, bearing several hard digits; but in other species it is more simple, and without the digits. The perfect Insect has no sound-producing organs, and it is very remarkable therefore to find the larvae {193}provided with highly-developed stridulatory structures. No auditory organ is known, unless the peculiar spiracles be such. [Illustration: Fig. 87—Head and prothorax of forms of the male of a stag-beetle; _Homocoderus mellyi_ (Africa). A, Large, B, intermediate, C, small forms. (From a photograph by R. Oberthür.)]