CIRCULATION 



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that it is scarcely conducive to the progress of physiological 

 knowledge to call two fluids with such different functions by one 

 name. The blood of Insects varies according to the species, and 

 in all probability even in conformity with the stage of the life 

 of the individual. Its primary office is that of feeding the 

 tissues it bathes, and it cannot be considered as having any aerat- 

 ing function. It is frequently crowded with fatty substances. 

 Graber says : " The richness of Insect blood in unsaponified or 

 unelaborated fat shows in the plainest manner that it is more 

 properly a mixture of blood and chyle ; or indeed we might say 

 with greater accuracy, leaving out of consideration certain 

 matters to be eliminated from it, that it is a refined or distilled 

 chyle." Connected in the most intimate manner with the blood 

 there is a large quantity of material called vaguely the fat- 

 body ; the blood and its adjuncts of this kind being called 

 by Wielowiejski ^ the blood - tissue. We shall return to the 

 consideration of this tissue after sketching the apparatus for 

 distributing the refined chyle, or blood as we must, using the 

 ordinary term, call it. 



There is in Insects no complete system of blood-vessels, though 

 there is a pidsating vessel to ensure distribution of the nutritive 

 fluid. This dorsal vessel, or heart as it is frequently called, may 

 be distinguished and its pulsations watched, in transparent 

 Insects when alive. It is situate at the upper part of the 

 body, extending from the posterior extremity, or near it, to the 

 head or thorax, and is an elongate tube, consisting as it were of 

 a number of united chambers ; it is closed behind, except in 

 some larvae, but is open in front, and has several orifices at the 

 sides ; these orifices, or ostia, are frequently absent from the 

 front part of the tube, which portion is also narrower, being 

 called the aorta — by no means a suitable term. Xear the lateral 

 orifices there are delicate folds, which act to some extent as 

 valves, facilitating, in conjunction with the mode of contraction 

 of the vessel, a forward movement of the blood. The composition 

 of the tube, or series of chambers, is that of a muscular layer, 

 with internal and external membranous coverings, the intima and 

 adventitia. Olga Poletajewa states " that in Bombus the dorsal 

 vessel consists of five chambers placed in longitudinal succession, 

 and not very intimately connected, and that there is but little 



" Zeitschr. wiss. Zool. -xliii. 1886, p. 512. ^ Zool. Anz. ix. 1886, p. 13. 



