The mermithids are preeminentl.v insect parasites although 

 crustaceans, spiders, snails, and some other invertebrates are 

 included among their hosts. Most of our knowledge regarding 

 life cycles and habits has beeu derived from a study of species 

 that infect insects and the following discussion has, of necessity, 

 been written with these hosts in mind. 



Eggs may hatch outside the host and larvae reach the body 

 cavity of the young insect by penetrating its body wall or eggs 

 may be ingested and larvae reach the body cavity by penetrat 

 ing the wall of the gut. In the former type of life cycle there 

 is a tendency for larvae to enter while their hosts are young 

 and for each host to harbor a small number of parasites. In 

 the latter type of life cycle the chances of the host becoming 

 infected are likely to increase with its age and food consump- 

 tion and the number of parasites per host is likely to be greater. 

 Tetradonema plicans, after reaching the body cavity of its 

 dipterous host, develops to maturity, copulates and lays its eggs 

 as an internal parasite. This is a simpler and probably a more 

 primitive life cycle than that known for any mermithid. Most 

 mermithids, after completing growth, force their way out of the 

 host and are free living during the adult stage. For Ae/amer- 

 mis decaiulata, Mcrmis subnigrcsccns, and probably some other 

 species, the free living period is of two years' duration and 

 during it the worms undergo their last molt, copulate, and fe- 

 males lay their eggs. Aproctotiema entomophagum develops to 

 maturity and copulates within its host but females emerge to 

 lay eggs, while Paramermis contorta undergoes its final molt 

 within the host but emerges before copulation. For both these 

 species the free-living stage is of very short duration and these 

 life cycles seem to represent intermediate steps between the 

 life cycle of Tetradonema pUean.i and that of such species as 

 Agamermis decaudata. 



If a species enters its host by penetrating the body wall the 

 posterior portion of the larva is often modified to serve as a 

 propelling organ. In some cases, as for example species of 

 Agamermis, this modified posterior portion, which may consti- 

 tute as much as four-fifths of the total body length, is de- 

 tached during the act of penetration and remains on the out- 

 side. In other species this modified portion is relatively shorter 

 and persists to form a horn-like appendage at the posterior ter- 

 minus of the fully grown larva. 



In most mermithids, especially those having an adult, free- 

 living stage of considerable duration, the intestine grows rap- 

 idly during parasitic development until it fills nearly all the 

 space in the body not occupied by other organs. This modified 

 intestine, filled with reserve nutrient materials and frequently 

 referred to as the "fat body," is largely responsible for the 

 opaqueness of the fully grown larva. The adult becomes in- 

 creasingly transparent as these stored nutrients are consumed 

 and life ends when they are exhausted. 



Most mermithids are represented by both sexes but the sex 

 ratio is subject to a good deal of variation, not only as between 

 different species but in the same species. Males of Amphimer- 

 mis suimiislii and of Agamermis deeandata considerably out- 

 number females while males of Mermis vigrescens and M. sub- 

 riigrescens are rarely found. The sex ratio of some species is 

 influenced by environmental conditions during parasitic develop- 

 ment. One or a few parasites per host results in a prepon- 

 derance of the larvae developing into females while a large 

 number of parasites per host results in all, or nearly all, devel- 

 oping into males. Convincing data demonstrating this environ- 

 mental influence on sex ratios have been presented by Caullery 

 and Comas (1928) for Parainermis contorta, by Christie (1929") 

 for Mermis subnigreseens, and by Kaburaki and Ij'atomi (1933) 

 for Amphimermis zuimusM. There is evidence suggesting that 

 some other species behave in a similar manner. 



Functional females that possess such male characters as cau- 

 dal papillae, male copulatory muscles, and even rudimentary 

 spicules have been reported from numerous species. It seems 

 probable that there is some correlation, as yet not understood, 

 between the influence of environment on sex and the occurrence 

 of these so-called "intersexes." 



Females of Hexamermis sp. (parasite of the ant, Fheidoie 

 pallidula) and of Agamermis decaudata lay eggs only after 

 copulation. Females of AUomermis myrmecophilia and of Mer- 

 mis subnigreseens produce viable eggs in the absence of males 

 though individuals of the latter species have been observed in 

 copula. 



The presence of mermithid parasites affects insects in various 

 ways; development of the gonads, especially the ovaries, is 

 usually suppressed resulting in sterility; wing muscles are some- 

 times weakly developed reducing ability to fly ; internal fat de- 

 posits are largely consumed; development of the body as a 

 whole may be retarded and metamorphosis delayed ; and infected 

 individuals may be .sluggish or, in the case of ants, have a 

 voracious appetite. As a rule external morphological characters 

 are not appreciably modified but there are exceptions, that of 



ants being the most outstanding. The emergence of the para- 

 site usually results in the death of the host. 



Numerous species of ants are rather commonly infected with 

 mermithids. Males, females, workers and soldiers have been 

 reported as harboring these parasites and there is wide varia- 

 tion in the effects of the mermithids on the external anatomy 

 of the hosts. In some instances infected ants show little recog- 

 nizable difference from normal individuals of the same sex or 

 caste, except, perhaps, a somewhat more distended gaster and 

 slight variations in color. This seems frequently to be the 

 case with infected males but sometimes, according to Gosswald 

 (1930) and Vandel (1934), infected females, workers or sol- 

 diers are not materially modified. In some instances, on the 

 other hand, the external anatomy is greatly modified (Fig. 169 

 C-G) and infected ants are not identical to any normal caste 

 but show female, worker and soldier characters in varying de- 

 grees. Such individuals are called intercastes. 



In the genus Lasius infected females resemble normal females 

 but are easily recognized, at least in many instances, by a 

 smaller head, shorter wings, and a somewhat more distended 

 gaster. Intercastes of this type have been designated mer- 

 mithogj/nes. 



In the genus Pheidole, Wheeler (192S) found a variety of 

 different intercastes with mixtures of soldier, worker and female 

 characters. He recognized five more or less distinct types 

 lias'.'d on thn degree of resemblance to one or another of these 

 three normal castes. In all these types the resemblance was 

 more especially to woikers and soldiers and for these intercastes 

 Wheeler proposed the term mermithergates. 



To Vandel (1930), working with Pheidole pallidula, the sit- 

 uation was somewhat simpler as he was able to recognize only 

 two types of intercastes. One type showed no very pronounced 

 difference from normal workers except a somewhat more dis- 

 tended gaster. The other type he believed to be modified sol- 

 diers and for these he proposed the term mermithostratiotes re- 

 serving the term mermithergates for those intercastes where 

 resemblance to workers predominates. 



Gosswald (1930) found young mermithid larvae in ants at 

 various times of the year and concluded that there may be con- 

 siderable variation in the time when these insects acquire their 

 parasites. Although mermithids have been found in larval ants, 

 only a few such cases have been reported, and Vandel concluded 

 that the infection is usually acquired during or just prior to 

 the pupal stage. Based on the size and development of larval 

 mermithids from young ants, Gosswald concluded that the para- 

 sites may be acquired when the immature insects are in differ- 

 ent stages of development and that the stage when the parasites 

 nre acquired determines, in a large measure, the degree to which 

 the adult host will be modified. 



How ants acquire these parasites is a question that has 

 aroused considerable interest but stimulated little actual inves- 

 tigation. Gosswald (1930) conducted infection experiments 

 with Lnsius alienus and used eggs of what was, presumably, 

 AUomermis myrmecophilia. His results indicate that the ant 

 acquires this parasite by Ingesting the eggs. As ant-infecting 

 mermithids belong to several genera (Agamermis, He.ramermis, 

 Allomerinis, etc.) life cycles and behavior undoubtedly differ 

 and all may not necessarily enter the host at the same time or 

 in the same manner. It would be surprising if an ant became 

 infected with a species of Agamermis by ingesting its eggs. 



Tetradonema plicans Cobb, 1919, is a parasite of the dip- 

 terous insect, Sciara coprophila Lint. It has been found in only 

 one collection of these insects made by Hungerford (1919) at 

 Manhattan, Kansas, in which every individual was infected. It 

 occurred in larval, pupal, and adult flies each insect harboring 

 from 2 to 20 parasites with an average of about 10, the number 

 of males slightly exceeding the number of females. T. plicans 

 passes its adult stage and lays eggs within its host, differing in 

 this respect from any mermithid of which the life history is 

 known. 



How the insects acquire their infection has not been deter- 

 mined. Eggs (Fig. 108 G) secured by Hungerford from around 

 females dissected out of fly maggots hatched in a few hours 

 when placed in water and the larvae that emerged seemed to be 

 identical with the youngest larvae found within the insects. 

 These larvae were of two types, a slender type about 12.')M long 

 with a curved caudal end and a plumper type about 90/^ long. 

 This difference, presumably, is sexual dimorphism. Hungerford 

 found eggs of the parasite in the digestive tract of small Sciara 

 larvae and concluded that eggs are probably swallowed and 

 nematode larvae, after hatching, penetrate through the wall of 

 the gut into the body cavity. He noted, however, that "the 

 older maggots are much less susceptible to infestation than the 

 younger ones" and he figures the tail of the adult parasite with 

 a horn-like projection whicli suggests that the larva has a cau- 

 dal propelling organ, two characteristics that one is inclined 



2.52 



