54 
Parasitism 
untimely end certain larvae of the small-lieaded kind are fed on pure 
saliva, and the protozoa disappear. The growth of the sexual organs 
which then takes place is probably due to relief from the pressure 
of the coecum, though the absence of certain specific secretions of the 
parasites is conceivably a factor. 
In a recent article on the “Polymorphism of Ants” Wheeler (1907) 
describes and discusses several cases of individuals occurring commonly 
in certain ant-colonies whose peculiar constitution he refers to “ trophic 
disturbances induced by specific parasites.” In the nests of the genus 
Pheidole very large and conspicuous ants attract the attention among 
the crowds of smaller workers and soldiers. They possess an enormously 
distended abdomen and attain to a length of 5 mm. (the workers do 
not exceed 3 mm.). Wheeler calculates that their volume is some 
twelve times that of the workers. Though not so markedly they also 
exceed the soldiers in length. 
The great enlargement of the abdomen is in all cases seen to be 
due to the presence of a parasitic worm, determined as a species of the 
nematode Mermis. Wheeler concluded as the result of dissection that 
it was a parasite of the alimentary canal and did not lie in the body 
cavity. The fat-body and alimentary canal had alike disappeared and 
nearly all the available space was occupied by the distended crop. To 
these singular individuals Wheeler gave the name of mermithergates. 
Infection must be effected while the future mermithergate is still 
a larva, but in pupal examples taken in association with adults the 
abdomen was not distended. The pupae were observed to hatch and 
at once the abdomen began to enlarge rapidly. The parasite is then 
of small size as this period begins but its speedy development is due 
to the supply of rich and abundant food to the newly hatched adult 
by the workers. Wheeler has graphically described the state of 
“chronic hunger” characteristic of the mermithergate and due to the 
growth of the parasite. 
It may be pointed out that it is not only the digestive sj^stem which 
is hypertrophied but the “ whole body exhibits excessive and uniform 
growth ” with the exception of the head. Whether we consider the 
mermithergate as a modified worker or soldier, we must recognise the 
small head as a sign of incompetence to produce the powerful mandibles 
and attached muscles of the other forms, possibly because the parasite 
is able to appropriate the special head-forming substauces. The huge 
head of the soldiers and some workers is a recently acquired character, 
and in some nests even scarcity of food appears to be responsible for 
