June 15, 1921 Life-History Studies of Three Jointworm Parasites 423 
There are 5 pairs of smali setae on the head—a pair just above the mandi¬ 
bles, a pair laterad of these, a pair dorso-mesad of the antennae, a pair 
caudo-dorsad of the antennae, and a pair on top of the head. The man¬ 
dibles (PI. 76, O) have changed shape considerably and are now much 
broader at the base and are less slender and less chitinized than in the 
previous instar. The labrum bears 4 heavily chitinized lobes or denticles. 
The body bears 9 pairs of spiracles, a pair being borne on each of the last 
2 thoracic and first 7 abdominal segments. The setae on the body in this 
instar are very small and inconspicuous. There are 2 subdorsal rows, 
2 lateral or spiracular rows, and 4 subventral rows on the thoracic seg¬ 
ments; also 4 additional subdorsal setae on the first thoracic segment and 
2 subventral setae on both the eighth and ninth abdominal segments. 
Third instar (PI. 78, E). —The color and general shape are very much 
as in the previous instar. Four larvae in this instar averaged 1.610 mm. 
in length and 0.4925 mm. in greatest width. The setae on the head are 
slightly more prominent. There are 5 pairs of setae located as in previous 
instar. Antennae as in previous instar. Mandibles (PI. 76, P) with 
general shape same as in previous instar, although they have increased 
considerably in size. The labrum in this instar has 4 prominent chiti¬ 
nized denticles or lobes and a rather inconspicuous one at each side of 
these. Spiracles as in previous instar. Between the last 2 thoracic, the 
last thoracic and first abdominal, first and second, and second and third 
abdominal segments there are rather prominent, fleshy, dorsal folds. 
The setae are slightly more prominent and are arranged as in previous 
instar, except for 1 additional subventral pair on first thoracic segment, 
and an additional pair of spiracular setae on the sixth and seventh ab¬ 
dominal segments. 
Fourth instar (PI. 78, F).— Color, general shape, head, antenna, and 
setae of head as in previous instar, except that the setae are much more 
conspicuous. Two larvae in this instar averaged 1.855 mm. in length and 
0.6300 mm. in greatest width. The mandibles (PI. 76, Q) in this instar 
are stouter and more heavily chitinized than in the previous instar. 
There are 4 heavily chitinized lobes to the labrum with an inconspicuous 
lobe on each side of them. Spiracles as in previous instar. The dorsal 
fleshy folds same as in the previous instar with an additional one between 
the third and fourth abdominal segments. This character is probably 
somewhat dependent upon the position the larva is in at the time of 
observation. The body setae are slightly more prominent than in the 
previous instar. There are 14 setae on the first thoracic segment, 8 on 
both the second and third thoracic segments; 4 setae on each of the first 
5 abdominal segments, 6 on each of the sixth, seventh, eighth, and ninth 
abdominal segments, 4 on the tenth. The setae are arranged as shown 
in Plate 78, F. 
