ex Introduction. 



organ, as in the reverse way the great development of the liver in 

 Arachnida may be supposed to compensate for their less developed 

 aerating organs. The other set of depuratory organs, those, namely, 

 which are charged with the direct elimination of effete nitrogenous 

 substances, are always, with the exception of Aphis, Coccus, and 

 Chermes, present in insects as the ' Malpighian vessels,' The tra- 

 cheae of insects inosculate or anastomose in various parts of their 

 course, but the capillary tracheae in which the spiral thickening of 

 the inner lining membrane may fail to be developed, end blindly in 

 the tissues to which they are distributed. The aquatic larva of one 

 insect, Cliloeon dimidiatum, has no tracheae developed in the first 

 three stages of its larval life, but subsequently, like the larvae of 

 many other Orthoptera, Neuroptera, and Diptera, has tracheae 

 developed which are exposed to the action of the oxygen dissolved 

 in outgrowths known as ' tracheal gills,' These organs do not ordi- 

 narily possess any ' spiracle ' whereby to come directly into commu- 

 nication with the air of the atmosphere, and must therefore obtain 

 the oxygen they contain as gas from that which is dissolved in 

 the water they live in. They may be said therefore to present us 

 with an instance of an arrangement transitional in character 

 between aquatic and aerial respiration. In a single insect, Ftero- 

 narcys regalis, one of the Orthoptera Amphibiotica, which is of 

 lucifugous habits, and inhabits damp localities, tracheal branchiae 

 are retained during adult life, but in other cases, when the insect 

 leaves the water^ the external lappets into which the internal 

 tracheae send ramifications fall off*, and the ordinary laterally- 

 placed spiracles are formed at the points of their separation. 



The heart is a vasiform organ, consisting ordinarily of eight 

 segments, with as many pairs of venous inlets. It underlies the 

 dorsal elements of the abdominal segments, is ordinarily closed 

 posteriorly, but ends anteriorly at the thorax in an aorta which 

 may be prolonged forwards as far as the cephalic ganglia. 



In Insects there is never wanting a ventrally- placed ganglionic 

 mass in addition to the first sub-oesophageal centre, which by its 

 commissural jimction to the cerebroid mass forms the nerve collar. 

 The first sub-oesophageal ganglia supplies the jaws, and though not 

 so closely apposed to the supra-oesophageal centres as is the case in 

 Arachnida, it is yet so close as often, but inconveniently, to have 

 been spoken of as part of the ' brain.' The ganglionic centres 



