4 6 ANTS. 



even for the mosi diminutive and soft-bodied s])ecies of Iridomyrmex, 

 Tapinoina. . Iztcca, etc. 



The Circulatory System. Janet ( 1902) has studied this system in 

 Mynnica. It comprises, as in other insects, the heart, aorta, h;emo- 

 lymph, or blood plasma, amcebocytes, or blood-corpuscles, and several 

 ductless glands of very simple structure. The heart (Fig. 15, lit. Fig. 

 24, c ) is a tube lying in the mid-dorsal region of the gaster and pre- 

 senting five dilatations corresponding with the first to fifth gastric 

 (fourth to eighth abdominal) segments, and each of these metameric 

 regions is pierced by a pair of osteoles provided with valves. The wall 

 of the tube is only a single cell-layer in thickness and the cells of its 

 two halves are in pairs, indicating that they arise from the pairs of 

 embryonic cells which I have called cardioblasts (1893). There is 

 no layer of muscles enveloping the tube, but very contractile muscle 

 fibrillae are differentiated in the cytoplasm of the cells themselves. The 

 tube is held in place by numerous suspensory filaments and five pairs 

 of so-called aliform muscles, belonging to the first to fifth gastric seg- 

 ments. These muscles are fan-shaped, with their broad ends meeting 

 and uniting in the middle line below the cardiac tube and their pointed 

 ends inserted on the supero-lateral walls of the gaster. Anteriorly the 

 heart is continued through the slender abdominal pedicel and into the 

 thorax as the 'aorta, a slender non-contractile tube which opens into 

 the head cavity. 



The blood, as in other insects, is a colorless liquid filling the body 

 cavity or spaces between all the internal organs and containing very 

 small, colorless, amoeboid and nucleated corpuscles. Circulation is 

 effected by the systole and diastole of the heart, the pulsations of which 

 proceed in a wave from its posterior to its anterior end. These move- 

 ments are described as follows by Janet, with the aid of the accompany- 

 ing diagram (Fig. 24, B } : " During systole the aliform muscles i am i. 

 the suspensory filaments (sf) and the heart (c) occupy the positions 

 represented by the unbroken lines. In contracting, the aliform muscles 

 shorten, and owing to this shortening, they recede in the middle region 

 from the dorsal integument and take the position represented by the 

 dotted lines. This movement draws down the suspensory filaments 

 attached to the muscles and changes the direction of those attached to 

 the dorsal integument. As these filaments can be but slightly elongated, 

 the changes of position here described are produced, so to speak, entirely 

 at the expense of the elasticity of the cardiac wall, which dilates consid- 

 erably. With this dilatation the valvules move away from the points to 

 which they were applied and the blood streams through the osteoles and 

 fills the heart. The blood, propelled by the contracting heart, pours 



