90 



NATURAL HISTORY OF ARTHROPODS. 



into thorax and abdomen exists. Lastly, in the mites all of the regions are more or 

 less confused and merged into each other. 



The number of segments which enter into the composition 

 of the head is variously stated, some authors admitting only 

 two, while others strive to recognize seven. It would, how- 

 ever, seem that there are in reality four cephalic segments, a 

 view which receives confirmation, according to Savigny's law, 

 in the number of appendages in this region, which are also four. 

 These appendages are a pair of antennas and three pairs (man- 

 dibles, maxillae, and labium) concerned in the preparation of 

 bn food for the mouth. In the simpler types of insects (as the 

 FIG. 125. Head of grass- grasshopper) these mouth-parts acquire their typical develop- 

 f /Eye! ment 5 in others ( as the % and butterfly) they become greatly 

 n. LabiTpaipus. ^oJem. modified, so that the study of their relations becomes a matter 

 ^Maxillary palpus, x. Max- o f some difficulty. In the spiders, if we regard all four of the 



walking feet as thoracic, there are but two pairs of appendages 

 which can be regarded as belonging to the head, the 

 chelicei'ae, or mandibles, and the palpi, or pedipalpi. 



The head also bears the eyes. In the spiders and 

 most myriapods only simple eyes or ocelli are found ; 

 but in most hexapods we have a pair of compound 

 eyes, and usually, except in most beetles, two or three 

 ocelli in addition are present. The antennae may 

 with great probability be also regarded as sense 

 organs, authors interpreting the minute structures 

 found upon them as either olfactoiy, auditory, or 

 tactile organs. 



The thorax, which exists as a distinct region only 

 in the Hexapoda, is there composed of three segments, 

 each of which bears a pair of limbs fitted for locomo- 

 tion (making six legs in all), and in addition each of 

 the two hinder segments usually supports a pair of 

 wings. These thoracic rings have their distinctive 

 names ; the first being the prothorax, the middle one the mesothorax, and the hinder 

 one the metathorax. 



Each thoracic leg is made up of a series of joints, and as each of these has a tech- 

 nical name of frequent use in all descriptive works, and, from necessity, in the following 



pages as well, we insert them here. The first joint, the 

 one which articulates with the body, is called the coxa ; 

 next conies another small joint, the trochanter; the thigh 

 or femur, a long and stout portion, is third ; the shank or 

 tibia, also long but more slender, conies next ; and, lastly, 

 we have the tarsus or foot, Avhich is made up of a number 

 of small joints arranged one after the other like the joints 

 of the human finger. The number of these tarsal joints 

 varies considerably, but the typical number in most hexapods is five. 



In the adult insects the appendages of the abdomen are used in locomotion only in 

 the myriapods. In the other groups, when present, they are much altered in form, and 



FIG. 126. Mouth-parts of grasshopper. 

 /. Labium. Ip. Labial palpus. tr. La- 

 bruiu. i. Mandible, mp. Maxillary pal- 

 pus, mx. Maxilla. 



FIG. 127. Typical insect leg. c. 

 Coxa. /. Femur. o. Trochauter. 

 r. Tarsus, t. Tibia. 



