2 SOUTH-AFKICAN BUTTERFLIES. 



ages exist in tlie Myriopoda) ; nor are the prevalent stalked or pedun- 

 culated eyes of Crustaceans to be found in the other classes, except 

 in a very few instances — and in these the stalked eyes are immovable. 



The' nervous system of the Arthropoda is situated inferiorly, con- 

 sisting of a double nerve-cord presenting a pair of swellings or nerve- 

 centres (ganglia) at intervals, the total number of which normally 

 corresponds to that of the segments, but varies in proportion as the 

 more or less intimate union of the segments in groups accompanies the 

 coalescence of certain ganglia. From the ganglia proceed nerves 

 extending to the various organs ; and, between the two pairs (superior 

 and inferior) of the head which constitute the " brain," passes the gullet 

 (oesophagus). The upper (" ccrelral ") of these two pairs supplies the 

 nerves of the antennee and eyes, and the lower (" cerebellar ") those of 

 the organs of the mouth. The three pairs of the thorax emit nerves 

 to the muscles of that region, and to the legs and wings (if any) 

 attached to it ; in some of the higher Insecta these thoracic ganglia 

 are combined to form a single nerve-centre, while in others the middle 

 and hind ganglia only are united, leaving the front pair apart. In 

 addition to this main system there is a second visceral one, also 

 ganglionated, which originates in the cerebral ganglia, and is distri- 

 buted to the gullet and stomach. 



These latter organs, with the rest of the alimentary canal and its 

 accessory glands, lie centrally along the body, above the main nervous 

 system, but beneath the circulatory or blood-vascular system ; which 

 latter consists of a long-chambered dorsal vessel or " heart," situated 

 along the middle line of the abdomen, and terminating anteriorly in a 

 thoracic aorta. 



The reproductive system is elaborately developed, and the sexes are 

 separate — except in the cases of certain low Arachnids (Tardigrada), 

 and of the probably degenerated Crustaceans known as Barnacles 

 (Cirripedia). The Arthropoda are with but few exceptions oviparous, 

 but some produce the larvee already hatched. 



Among the three classes of air-breathing or trachcated Arthropoda, 

 the Insecta are, as their name implies, specially distinguishable by the 

 very marked division of the body into the three separate portions of 

 head, thorax, and abdomen. In the Arachnida the head and thorax 

 coalesce into one mass (cephalo-thorax), while in the Myriopoda the 

 thoracic and abdominal segments exhibit no distinctly separate grouping. 

 Insects and Arachnids agree in never having any jointed limbs attached 

 to the abdominal segments ; but the former never have more than six 

 (three pairs) ambulatory or walking limbs, while the latter have eight ; 

 and insects alone in the sub-kingdom are provided with wings. These 

 organs are not true limbs like the hollow jointed legs, but merely 

 expansions of the integument, springing from the sides of the middle 

 and hind segments of the thorax ; they are traversed, and at the same 

 time extended and supported by hollow, horny, stiff, rib-like tubes, 



