June 21,1924 
Morphology of the Honeybee Larva 
1195 
in number and arrangement from the trunk muscles of the larva that identifica¬ 
tion of any given muscle or set of muscles of the imago with those of the larva 
proves to be impossible without a knowledge of the intervening stages. 
The general features of the histology of the larval muscles has already been 
given (36). This description applies to the newly hatched larva, but no essen¬ 
tial changes take place during larval growth. In this account it is stated that 
the muscles of the young larva are not striped. However this may be, in the 
old larvae cross-striped muscles are the rule. Mention should be made of the 
constant occurrence of very distinct fibrillae, “tomomitosomes,” Maziarski (34), 
“filaments de resistance,” Janet (20, p. 54), in the hypodermal cells at the points 
of attachment of muscles. These fibrillae frequently assume a brush-like form, 
as shown in Plate 6, D, at both of the ends of the dilator of the pharynx and at 
the posterior ends of the mandibular muscles. The filaments bind muscle, 
hypodermis, and cuticle firmly together, as shown in fixed material by the cling¬ 
ing of the cuticle, elsewhere loosened, to the hypodermis at the points where 
muscles are inserted. 
HEART AND BLOOD CELLS 
HEART 
The heart of the larva is in most respects like the less differentiated portion 
of the imaginal heart which is situated in the abdomen. The heart consists 
essentially of a slender thin-walled tube situated in the mid-line close beneath 
the dorsal hypodermis. It is widest (about 0.25 mm.) at its posterior end 
and gradually diminishes in caliber up to the anterior border of the 2d trunk 
(the mesothoracic) segment. Here the heart leaves the dorsal body wall and 
bends downward, passing beneath the anterior tracheal loop (PI. 3, A, Ao ), at 
the same time diminishing rapidly in diameter, and is continued cephalad as 
the aorta. The aorta is not strictly tubular but is open on the ventral side, hav¬ 
ing in transverse section the form of an inverted letter U, the free edges hanging 
down on each side of the oesophagus and becoming clothed on the exterior with 
a layer of tracheoles. At the posterior side of the brain the aorta becomes 
reduced in its dorso-ventral diameter to enter the narrow cleft between the 
oesophagus and the upper ends of the crura cerebri and finally terminates at 
the anterior face of the brain. The posterior end of the heart terminates blindly 
in the 9th abdominal segment. In life the heart is transparent, and since it is 
bounded on either side by the relatively opaque white fat cells, it produces 
externally the appearance of a dark band along the dorsal mid-line of the larva. 
At the middle of trunk segments 2 to 11, inclusive, the heart is sharply con¬ 
stricted and is thus divided into 11 chambers. These constrictions, however, 
do not affect the dorsal and ventral walls of the heart, but only the lateral walls, 
which are in fact indented by a series of pairs of opposite V-shaped indentations 
the open ends of which are directed slightly caudad. At the bottom of each 
indentation is a linear slit; these slits constitute the ostia (fig. 5, B, Ost). The 
ostia are not, however, precisely normal to the long axis of the heart, but are 
slightly oblique, their dorsal ends being slightly caudad of their ventral ends. 
The action of the heart is simple and similar to the heart action of many 
other insects. The heart walls bordering on the ostia form valvelike flaps 
projecting inward which allow a free inrush of blood during diastole, but which 
automatically close the ostia during systole. The posterior pair of flaps project 
inward far enough also to close the posterior ends of the heart chambers dur¬ 
ing systole, preventing a backward flow of blood. Moreover, the thickened 
margins of the ostia share in the contractility of the heart wall, so that probably 
these also possess the ability to contract the ostia and the posterior ends of the 
heart chambers as well, thus acting as sphincter muscles. 
