INSECTA 439 



Further, in certain insects accessory hearts are present which assist 

 in the circulation through special regions (in the thorax of the beetle 

 Dytiscus and in the bases of the legs of Aphids where they propel blood 

 through the wings and legs of these forms respectively). The much 

 reduced system is on the whole greatly in contrast with the complex 

 arrangements of the decapod Crustacea and of such Arachnids as 

 Limulus and the Scorpions where the respiratory pigment haemo- 

 cyanin renders the blood of the greatest importance in respiration. 

 The part played by blood in respiration introduces a topic which can 

 only adequately be considered with the tracheal system next to be 

 described. In anticipation of that account it may suffice to note the 

 following points. The walls of the tracheae are freely permeable to 

 gases and there must therefore occur an exchange of gases between 

 the blood and the air in the air in the tracheae. In some insects the 

 walls of air sacs within the tracheal system become intucked so as 

 to form "inverted tracheae" through which blood circulates, thus 

 giving rise to an organ which may act as a veritable lung, e.g. Sphinx 

 and Crabro. Though these facts suggest a special oxygen-carrying 

 function for the blood, it appears that its oxygen capacity is no greater 

 than can be accounted for by physical solution. Haemocyanin does 

 not occur and to this fact must be put down the rather vestigial nature 

 of the blood system in insects. Haemoglobin occurs in a few, e.g. the 

 larva of the midge Chironomus, the male apparatus of the water bug 

 Macrocorixa and in certain tracheal cells of the horse-fly Gastro- 

 philus. This pigment may be derived from intracellular cytochrome 

 and its occurrence be of the nature of a chemical accident of little 

 functional significance. On the other hand it may serve, as it appears 

 to do in Chironomus, as a means of enabling the animal to utilize 

 oxygen when this occurs only at low tensions in the surrounding 

 medium. The occurrence of chlorophyll invariably owes its origin to 

 the food plant. Of the several kinds of blood cells which exist, per- 

 haps those which play an important part in the histolysis of larval 

 tissues during the pupation of holometabolous insects, e.g. the blow- 

 fly Calliphora, are of most interest. 



Associated with the blood are the following cellular tissues, the 

 fat body, the nephrocytes, the oenocytes, the corpora allata, and in 

 various beetles, the photogenic organs. The fat body consists of 

 closely adherent cells, in the vacuoles of which products of 

 digestion are stored up. Fats, albuminoids and glycogen occur in this 

 way. In addition are found urates showing that this organ serves for 

 excretion. Oenocytes occur as bunches of large cells close to the 

 spiracles in the abdomen and develop as hypodermal invaginations 

 in these places. The corpora allata arise similarly in the mandibular 

 segment and subsequently come to lie above the oesophagus behind 



