June 21, 1924 
Morphology of the Honeybee Larva 
1197 
The wall of the heart is exceedingly thin, as shown by the figures, and is 
composed of a double row of cells, corresponding to the cardioblasts of the 
embryo, the two rows constituting the right and left sides of the heart, re¬ 
spectively. These cells have the form of the half of a radially compressed ring 
(Pl. 7, B). At the middle of its length each cell is thickened and at this point a 
large discoid nucleus is found. Traversing each of the heart cells lengthwise,, 
that is, at right angles to the long axis of the heart and mesiad of the nucleus, 
is a bundle of delicate fibrils, few in number, usually visible only under high 
magnification. They are most readily seen in cross sections of the heart cells, 
that is, in longitudinal sections of the heart itself, which have been counter- 
stained with eosin, the muscle fibrils of the heart cells, like those of other muscles 
in the larva, readily taking this stain. Their appearance in section is shown in 
figure 5, B, where they are represented in black. An attempt to represent 
these in lateral view is made in Plate 6, E, but they are actually less evident 
than here represented. In suitably stained preparations this bundle of fibers is 
plainly seen to be transversely striated (PI. 6, F). The heart cells are therefore 
muscle cells, essentially similar structurally to other muscle cells in the bee larva, 
for example the trunk muscles, but in which the fibrillae (myofibrils) are more 
feebly developed. Similar conditions of structure are found in the cells of the 
heart wall of the larva of Tipula ( 48) and of those of Vespa ( 1) and Aeschna 
(55). Since the heart of all insects is contractile, it seems probable that further 
investigation will show such a differentiation to be general. In the larva of the 
honeybee, the myofibrils may easily escape notice, since larvae stained to show 
other structures to the best advantage scarcely show them at all. A deep and 
heavy stain with iron haematoxylin is necessary to reveal the myofibrils in face 
view; that is, in tangential sections of the heart. The arrangement of the heart 
cells in a double row is evidently not unique, since in addition to being charac¬ 
teristic of insect embryos it has been described for the Tipula larva by Viallanes 
and for the Aeschna larva by Zawarzin (55). 
The heart is clothed externally by a loose meshwork of minute branched con¬ 
nective cells, the meshes tending toward elongation in a longitudinal direction 
(PI. 6, E and F, ConCls). This meshwork serves to strengthen the heart wall 
and also to anchor the heart in position, since many of the processes of the connec¬ 
tive tissue cells extend out to the hypodermis and to the adjacent fat cells. 
DORSAL DIAPHRAGM 
The dorsal diaphragm of the larva is well developed only in the posterior region, 
from the 4th to the 9th abdominal segments, inclusive, and here it is very similar 
in structure to that of the imago. In these segments it forms a continuous 
sheet, attached to the heart on its ventral surface (PI. 7, A, B, DDph) and extend¬ 
ing laterad, parallel with the dorsal body wall on either side of the dorsal mid¬ 
line, a distance somewhat less than one-eighth of the entire circumference of the 
larva. It partitions off a narrow dorsal space, the dorsal sinus or pericardial 
cavity. The lateral margins of the dorsal diaphragm are free except where they 
cross the lines of separation of the segments, and here the margins are attached 
to the body wall. Between these points the membrane is somewhat retracted 
mesiad, giving the margins a wavy contour (PI. 7, A). The dorsal diaphragm 
terminates in the anterior half of the 9th abdominal segment with a free border. 
Anglas (1) states that the two delicate membranes ensheathing the diaphragm 
cells are reflected in the median plane below the heart to form a sort of septum 
which does not reach ventrad to the mid-intestine. This membrane, or septum, 
which is termed the “cloison m^diane,” is said to separate the fat body into two 
symmetrical halves. This account, as regards the bee larva, appears to be incor- 
