like. An outline of the stoma appears at the head end, and a 

 conspicuous caudal vesicle and outline of the pyriform rec- 

 tum at the posterior end, but throughout the rest of the body 

 the nuclei are still scattered without definite order. Gradually 

 during the next few days the worm elongates, and the alimen- 

 tary canal, nerve ring, and rectum become well developed. 

 Meanwhile the tissue of the wall of the Malpighian tubule 

 surrounding the larva degenerates and is finally reduced to a 

 mere membrane, which serves as a sheath. On the eighth 

 day, at about the time of emergence of the adult fly, the 

 larvae begin to break loose into the abdominal cavity, still 

 enclosed in the membrane, but they now molt a second time 

 and their movements become very active, resulting in their 

 soon freeing themselves. 



These third-stage larvae, the infective forms, may appear 

 as early as the ninth day. They migrate forward to the 

 head of the fly, and collect in the interior of the labium. 

 Attracted by warmth and moisture they move down into the 

 labellum, and escape through the delicate membrane between 

 the lobes of this structure when the fly is resting on a warm 

 wet surface, e.g., the lips, nostrils or wounds of an animal. 

 If on the lips, the larvae have an opportunity to reach the 

 stomach via the mouth, and grow to maturity in a normal 

 manner, but from the nostrils they reach the lungs, and from 

 the skin the subcutaneous tissues, and in either case fail to 

 grow to maturity. There is no doubt but that animals 

 could also be infected by swallowing flies harboring infective 

 larvae, but in the case of habronemiasis of horses this would 

 probably not be a common method in nature. On the other 

 hand it would probably be the principal if not the exclusive 

 method in the case of habronemiasis of insectivorous birds. 

 Still another possibility — ingestion by a transport host — is 

 suggested in the case of habronemiasis in birds of prey ; 

 this is supported by the finding of abundant larvae of H. 

 mansioni encysted in the stomach walls of toads by Hsii and 

 Chow (1938). Tliis species had previously been recorded 

 from the bearded vulture, Gypaeiii.'i barbatiis, but several spe- 

 cies of falcons were experimentally infected by feeding them 

 larvae from toads. 



Habroneina muscac and H. micrn.ttomiim have similar life 

 cycles {vide Roubaud and Descazeaux, lB22a), but different 

 in details. These two species, instead of undergoing devel- 

 opment in the Malpighian tubes, develop in cells of the fat 

 body, the thickened walls of the cells serving ns temporary 

 "cyst" walls. H. micro.stomum. which develops in the blood- 

 sucking Stomoxys, might be expected to be introduced into the 

 tissues when the insect pierces the skin, and be forced to 

 find its way to the stomach by some roundabout parenteral 

 route, but Roubaud and Descazeaux (1922b) point out an in- 

 teresting biological adjustment which makes this unnecessary. 

 They point out that, as a result of interference by the worms 

 in its proboscis, the fly is unable to rasp a hole in the skin 

 to suck blood, and is forced to revert to the habits of its an- 

 cestors and non-blood-sucking relatives, and obtain moisture 

 and nourishment from the lips or other exposed moist sur- 

 faces. 



The failure of the larvae of Habroneina to become encysted 

 in the intermediate host, there to remain until eaten by a 

 definitive host, and the substitution of a voluntary exit from 

 this host in response to warmth and moisture, are definite 

 steps in the direction of a filarial life cycle. As remarked 

 by Roubaud and Descazeaux (1922b), however, the habrone- 

 mas are imperfectly adapted for parenteral parasitic life. Their 

 larvae, in spite of the fact that they leave the body of the 

 intermediate host on the surface of the body of the definitive 

 host, are unable to penetrate the tissues, and are unable to 

 reach maturity outside the alimentary canal. With (1) de- 

 velopment of a parenteral adult habitat (already attempted 

 by many spiruroids but always hampered by the necessity for 

 the eggs to reach the alimentary canal), and (2) develop- 

 ment of ability to enter the skin on the part of the infecting 

 larvae, the only important change necessary to bring about 

 a filarial life cycle is the substitution of the blood or skin for 

 the alimentary canal as a means of exit for the larvae. Such 

 a development could hardly fail to occur in the case of a 

 parenteral parasite with a blood-sucking intermediate host. 



Other, Spiruroidea 



The life cycle of the ma,iority of the Spiruroidea in which 

 it has been determined conforms in general pattern to that of 

 Gongylonema, except for the intermediate hosts involved. In 

 some cases there seems to be far less specificity with respect 

 to intermediate hosts than in others, but some instances of 

 apparent specificity are probably due to incomplete data. 

 Thus Cheilospirura hamulosa was not known to develop in any- 

 thing but grasshoppers until Alieata (1937) showed that an 

 amphipod and 10 species of beetles belonging to 7 different 



families, as well as several grasshoppers, could b? utilized as 

 intermediate hosts by this worm. On the other hand, Cram 

 (1931) got negative results from feeding eggs of C. spinosa 

 to cockroaches, ground beetles, sowbugs and crickets, but ob- 

 tained development in two species of grasshoppers. Again, 

 whereas Telramercs fissispina is reported as capable of devel- 

 opment in grasshoppers, roaches, Daphnia, Gainmariis and earth- 

 worms. Swales (1936) found that the eggs of T. crami failed 

 to hatch in various species of Cladocera, but developed read- 

 ily in two species of amphipods. Members of the genera 

 Ascarops, Physocepltaliis and Spirocerca seem to develop pri- 

 marily in dung beetles ; Spirura, Protospirura and Gongy- 

 lonema in beetles or roaches; Oxyspirura in roaches; Seurocyr- 

 nea in roaches and grasshopper nymphs; Acnaria in grass- 

 hoppers; Tetrameres in various Orthoptera and Entomostraca ; 

 Eartertia in termites (workers); Echinuria in Cladocera, 

 Dispharynx and Hedrnris in isopods ; Cystidicola in amphipods, 

 and Spiroxys in eopepods. Spiruroid larvae, possibly Protos- 

 pirura, have been found in fleas also. Under experimental con- 

 ditions Physaloptera iurgida, according to Alieata (1938), is 

 able to develop in cockroaches, but there is a possibility that 

 other arthropods are utilized under natural conditions. 



Spiroxys contorta, as reported by Hedrick (193.5), differs 

 from the majority of the Spiruroidea but resembles Gnatho- 

 stoma in that the eggs become embryonated in water after 

 leaving the body of the host. It differs from Gnathostoma, 

 however, in that the definitive host can be infected directly 

 by the third-stage larvae in Cyclops, without requiring a 

 second intermediate host. In nature, however, transport hosts 

 — fish, tadpoles, frogs, newts and dragonfly nymphs, and fre- 

 quently turtles as well — are commonly made use of. The 

 lar%-ae of this worm are further peculiar in that they continue 

 to grow after they reach the infective stage, both in Cyclops 

 and in the various transport hosts. The development of a 

 "sausage" form by the late first-stage larva of Oxyspirura 

 mansoni, as figured by Kobayashi, is highly suggestive of 

 Habroncma or the filariae. 



As far as known at present Giialkostoma spiiiigenim is the 

 only spiruroid which requires a second intermediate host, but 

 it is quite possible that this will be found to be true of other 

 Gnathostomatidae as well, and perhaps of still other spiruroids. 

 The larvae of Echiiiocepliahis (family Gnathostomatidae) have 

 been found encysted in the tissues of a bivalve, Margaritifera 

 vulgaris, which is presumably the first intermediate host. Simi- 

 lar larvae have been found in a sea urchin. Since the adults 

 occur in oyster-eating fishes no second intermediate host may 

 be necessary. 



The course of migration in the definitive host is usually, 

 as noted above, by burrowing directly through tissues or 

 natural cavities, or by migration along natural passageways. 

 The path of Oxyspirura mansoni to the eye, according to 

 Fielding (1926), is by way of esophagus, mouth and lachrymal 

 duct, the larvae sometimes arriving in the eye 20 minutes after 

 infected roaches are fed to chicks. 



The migration route of Spirocerca hipi (—sanguinolenta) 

 is not so clearly known. Faust (1927) thought that the larvae, 

 after ingestion with the flesh of a transport host (hedgehog), 

 reach the aorta via the portal system and lungs, but does not 



Fig. 192. 



Development of Ascaropsiniie larvae. A-E — Ascarops strons^ylina 

 (A — First st.age larva, anterior end, lateral view; B — Larva recovered 

 from an intermediate ho.st three days after e.xperiinental infection; C — 

 Larva undergoing first molt: D — Third stage larva, lateral view; E — 

 Encysted larva, third stage). F-K — Fhysocephtilus sexakttus (F — An- 

 terior end. lateral view; G — Larva from intermediate host 2 days after 

 experimental infection; H — Larva from intermediate host 12 days after 

 experimental infection; I — Larva undergoing first molt; J — Encysted 

 third stage larva (from Hobmaier, 1925); K — Third stage, tail). After 

 Alieata, 1935, U.S.D.A., Tech. Bull. 489. 



Fig. 193. 



A-G & G — Spiroirtjs contortus; (A — Free-living larva with sheath; 

 B — Five-day old larva from cyclops; C — Ci/clops leucknrti with three 

 larval nematodes: G — Fully developed larva from body cavity of cyclops. 

 showing genital primordium). O-E — Disphnrtni.r spiriilis (D — Head; 

 E — Tail). F — Tetrampres atuerirana, tail of third stage larva. H-I — 

 Tetrameres crami <H — Third stage larva from Oammarus fa-scintus 32 

 days after infection: I — Diagrammatic illustration of papillae ^on 

 tail of third stage larva). J — Larval spirurid larva from cat flea. K-M 

 — Protospirura. muricnla (K — Lateral view of anterior extremity of in- 

 fective larva; L — Lateral view of tail of 3.5 mm. specimen; M — Free- 

 had sketch of rosette of papillae on tail of same). N-P — Oxyspirura 

 mansoni (N — Larva just after hatching; O — Larva at end of first lar- 

 val stage; P — Mature larva). Q — Habronema mansioni, larva. A-C, & 

 G, after Hedrick. L. A., 1935, Tr. Am. Mic, Soc. v. 54(4). D-F, after 

 Cram, E. B., 1931, U.S.D.A. Tech. Bull. 227 H, I, after Swales, 1936, 

 Canad. J. Ees. D. 14. J, after Alieata, J. E., 1935, J. Parasit. v. 21 

 (3). K-M, after Foster, A. 0., and Johnson, C. M.. 1939, Am. J. 

 Trop Med. v. 19 (3). N-P, after Kobayashi, H., 1928, Taiwan Igakk. 

 Zasshi Formosa, No. 280. Q. after Hsu, H. P., and Chow. C. Y., 1938, 

 China Med. J. Suppl. II. 



286 



