Relationships of Insects

Abstract

The arthropods form the largest group in the animal kingdom and can be recognized by the following characters. The body is segmented and covered by a chitinous exoskeleton. A variable number of the segments carry paired, jointed appendages that exhibit functional modifications in different regions of the body. The heart is dorsal and is provided with paired ostia, a pericardium is present, and the body-cavity is a haemocoele. The central nervous system consists of a supra-oesophageal centre or brain connected with a ganglionated ventral nerve-cord. The muscles are composed almost entirely of striated fibres and there is a general absence of ciliated epithelium. The earliest known arthropods, the fossil group of Trilobita, were aquatic, but among living forms it is only the Crustacea that have retained this mode of life to a predominant extent. No other group of invertebrate animals has so large a proportion of its members terrestrial in habit.