Apr. 19, 1924 
Molts of the Ox Bots 
273 
of the first-stage larva are straight or only slightly curved, with a somewhat 
blunt point and without a large and heavily chitinized base. The maximum 
length of the spines is more than twice the maximum diameter at the base. 
The triangular spines on the border of the posterior spiracles are well pointed, 
more heavily chitinized, and longer than the other spines on the terminal segment. 
In the second stage the spines are very sharply pointed, strongly curved, and 
only slightly longer than the large, circular, elevated base. The shape of these 
spines, in every respect, is distinctly different from that found in spines on the 
first stage. The triangular spines on the border of the spiracles are equal in 
length or slightly shorter than the longest spines on the terminal segment. They 
are without a distinct point and shorter than in the first stage. The diameter 
of the spiracles has increased considerably compared with that of the first stage. 
In the third stage the spines on the terminal segment again show a striking dif¬ 
ference from those of the second stage and the greatly increased dimensions are 
in a reverse order. The diameter of the enlarged base, nearly three times that 
of the second stage, is now greater than the length of the spine, which itself has 
attained a length almost twice that of the second stage. The posterior spiracles 
have increased their diameter considerably and the triangular spines on the 
border of the spiracles have decreased in size to almost half that of the preceding 
stage and are now very blunt, or sometimes slightly knobbed at the point. 
Gedoelst, 3 who holds that there are only three stages in the larva of Hypo- 
derma, states that when growth takes place within the duration of the same 
stage it comprises all the soft parts of the organism, but that it does not affect 
the highly chitinized organs, such as the stigmata, cephalopharyngeal skeleton, 
and spines, which do not acquire any greater dimensions, except on the occasion 
of molts. He attributes the greater distance between the spines of the larger 
(second-stage) larvae to the growth of the soft parts of the organism, and ex¬ 
plains the vastly greater dimensions of the spines on the terminal segment as a 
continuation of the processes of chitinization. 
The greater distance between the spines of the second-stage larvae could 
easily be the result of growth of the soft parts of the organism but in such case 
the number and size of the spines should remain constant. This, however, is 
not the case. ‘Certainly the increase in the number of spines could not be 
explained as an extension of the processes of chitinization. By this theory it 
would seem even more difficult to explain the absence of spines on the body 
segments of the third-stage larvae. Koch, 4 who also holds that there are only 
three larval stages in Hypoderma, advances the theory that the spines on the 
body segments are gradually worn down as the larvae migrate through the tissues. 
This would contradict Gedoelst’s theory of the continuation of the processes of 
chitinization as the cause of the enlargement of the spines. Koch’s assump¬ 
tion was not correct, however, at least not in so far as the first and second-instar 
larvae are concerned, as the spines of the second-stage larvae, although actually 
slightly smaller in diameter, with the exception mentioned before, are much 
longer than in the first stage. 
It is not likely that the clearing process in preparing the specimens for slide 
mounts could be responsible for the disappearance of the spines on all the body 
segments in the third-stage larvae and not affect the small spines in the group 
below the mouth or any of the spines in second-stage larvae. 
Further evidence that there is a molt between the smaller spiny larvae and 
the larger spineless larvae is the fact that occasionally a larva is found that 
possesses One or sometimes several small groups of highly chitinized spines on 
3 Gedoelst, L— le trimorphisme larvaire des oestridSs. Compt. Rend. Soc. Biol. [Paris]86: 
501-504. 1922. 
4 Koch, T. P.— om oksebremsen hypoderma bovis. Maanedsskr. Dyrlaeger 15: 129-159. 1903 
