ARTHROrODA. 
291 
concentrated than in the preceding order. There are gen¬ 
erally eight simple eyes, rarely six. They breathe both 
by tracheae and lung-like sacs, from two to 
four in number, situated under the abdomen. 
All the species are carnivorous. 
The instincts of Spiders are of a high 
order. They are, perhaps, the most wily of 
Articulates. They display remarkable skill FiG. 26 i.-Spm- 
and industry in the construction of their neretsoftheSpi- 
webs; and some species (called “ Mason Spi- piform organs, 
ders”) even excavate a subterranean pit, line it with their 
silken tapestry, and close the entrance with a lid which 
moves upon a hinge. 151 
Class IV.—insecta. 
Insects are distinguished by having head, thorax, and 
abdomen distinct, three pairs of jointed legs, one pair of 
antennae, and generally two pairs of wings. The number 
of segments in the body never exceeds twenty. The head, 
apparently one, is formed by the union of four segments. 
The thorax consists of three — tli e prothorax, mesothorcix , 
and metathorax —each bearing a pair of legs; the wings, 
if present, are carried by the last two segments. The ab¬ 
domen is normally composed of ten segments, more or less 
movable upon one another. The skin is hardened with 
chitin, and to it, as in all Arthropods, the muscles are at¬ 
tached. The organs of sense are confined to the cephalic 
division of the body, the motor organs to the thoracic, and 
the vegetative to the abdominal. All the appendages are 
hollow. 
The antennas are inserted between or in front of the 
eyes. There is a great variety of forms, but all are tubu¬ 
lar and jointed. They are supposed to be organs of touch, 
and also seem to be sensitive to sound. The eyes are 
usually compound, composed of a large number of hexago¬ 
nal corneae, or facets (from fifty in the Ant to many thou- 
