266 Journal of Nematology, Volume 13, No. 3, july 1981

8. Klein, M. G., W. R. Nickle. P. R. Benedict, logical control of insects. Boca Raton, Florida: and D. M. Dunbar. 1976. Psammomermis sp. C.R.C. Press. (Nematoda: ): A new para- 14. Poinar, G. O., Jr., and G. G. Gyrisco. 1962. site of the Japanese beetle, Popillia japonica Studies on the bionomics of Hexamermis arvalis (Coleoptera: Scarabaeidae). Proc. Helm. Soc. Wash. Poinar & Gyrisco, a mermitbid parasite of the 43:235-236. alfalfa weevil, Hypera postica (Gyllenhal). J. Insect 9. Nickle, W. R. 1974. Nematode infections. Pp. Path. 4:469-483. 327-376 in G. E. Cantwe]l, ed. Insect diseases. New 15. I'oinar, G, 0., Jr., and H. E, Welch. 1968. A York: Marcel Dekker. new nematode, Filipjevimermis leipsandra sp. n. 10. Nickle, W. R. 1978. On the biology and life (Mermithidae), parasitic in Chyrsomelid larvae history of some terrestrial mermithids parasitic on (Coleoptera). J. Invert. Pathol. 12:259-262. agricultural pest insects. J. Nematol. 10:295 (Abstr.). 16. Polozhentsev, P. A. 1952. New Mermithidae 11. Nickle, W. R., and P. Grijpma, 1974. Studies of sandy soil of pine forests. Trudy Helmintb. Lab. on the shootborer, Hypsipyla grandella (Zeller) 6:376-~82. (Lep.. Pyralidae) XXV. Hexamermis albicans (Sie- 17. Wheeler, W. M. 1928. Mermis parasitism and bold) (Nematoda: Mermithidae) a parasite of the intercastes among ants. J. Exp. Zool. 50:165-237. larva. Turrialba 24:222-226. 18. Welch, H. E. 1963. Nematode infections. Pp. 12. Poinar, G. O., Jr. 1975. Entomogenous 363-392 in E. A, Steinhaus, ed. Insect pathology. . Leiden: E. J. Brill. New York: Academic Press. 13. Poinar, G. O., Jr. 1979. Nematodes for hip-

Mermithid Nematodes: Physiological Relationships with their Insect Hosts 1 Roger Gordon ~ Abstract: This paper assesses our state of knowledge of physiological processes involved in the relationships between insects and their mermithid nematode parasites. Three major components of the host-parasite relationship(s) are reviewed: effects of mermithids on host physiology, effects of host physiology on mermithids, and the physiology of the nematodes themselves. Mermithids induce an array of changes in host physiology, and the effects on host metabolism and endocri- nology are discussed at some length. Few studies have been done to ascertain the effects of the host on the parasites from a physiological standpoint. Whereas host immunity mechanisms against mermithids have been described at the ultrastructural level, the physiological basis of such responses is not known. Mermithids are atypical nematodes, both structurally and physio- logically. In the absence of a functional gut, nutrients are absorbed across the outer cuticle and stored in a trophosome. The transcuticular mode of feeding, storage within the trophosome, and metabolism of storage products are discussed. The usefulness of physiological information toward expediting in vitro culture of these nematodes is discussed, and problems that need to be ad- dressed are defined. Key words: cuticle, fat body, Filipjevimermis leipsandra, Gastromermis boophthorae, hemolytnph, immunity, , mermithid nematode, Neomesomermis flumenalis, Romanomermis culicivorax, trophosome.

Mermithid nematodes (Enoplida: Mer- sterile. The potential of mermithids for mithidae) have considerable potential for insect biocontrol, however, has yet to be biocontrol of insect pests. The insect host realized. Only one species of mermithid, the invariably dies when the juvenile nematode mosquito parasite Romanomerrnis culici- completes its parasitic development and vorax, has been mass cultivated on the scale exits from the host's hemocoel (the para- needed to permit field trials and for this an site's microenvironment). When adult fe- in vivo procedure was used (29). Several male insects are infected, they are rendered authorities (28,46) have advocated the es- tablishment of in vitro cultivation pro- Received for publication 9 February 1981. cedures because they are potentially 1Symposium paper presented at the annual meeting of the Society of Nematologists, New Orleans, Louisiana, Au- cheaper, more efficient, and easier to main- gust 1980. tain than in vivo methods. However, at- 2Department of Biology, Memorial University of New- foundland, St. John's, Newfoundland, Canada AIB 3X9. tempts to culture mermithids in vitro (8) Continuing financial support from the Natural Sciences /lave met with limited success. and Engineering Research Council of Canada (Grant No. A6679) is gratefully acknowledged. The paucity of information about the Mermithids--Physiological Relationships: Gordon 267 physiological interrelationships between in- tological examinations of chironomids: sects and mernaithids has hampered progress "Very little or no food is abstracted from of in vitro culture of mermithids. Mer- the haemolymph of the host, for in the mithids are atypical nematodes. They lack a parasitized forms there is always an increase, functional gut and absorb nutrients from instead of a decrease, in the lipoid content the host's hemolymph through their outer of the blood." Rempel's contention that the cuticle. The parasitic juveniles store these hemolymph does not constitute the primary nutrients in a storage organ (trophosome) source of nutrients for the developing mer- that almost completely fills the pseu- mithid has since been shown to be incorrect. docoelom. Storage reserves are utilized by Whether or not blood lipid levels really the free-living stages which feed only min- were elevated cannot be ascertained from imally or not at all. Such adaptations sug- his study, but such art increase would not gest that mermithids are deeply committed discount the hemolymph as a nutrient to parasitism and rely heavily upon their source. host to provide nutritional requirements Among mermithids, as among nema- and, in all probability, stimuli for growth todes in general, there is considerable uni- and development. For in vitro mermithid formity in morphology and general pattern culture to be effective, these nutrients and of life cycle. It would seem reasonable to stimuli have to be identified and incorpo- suggest that information obtained concern- rated into the culture media. ing the physiology of one species would, in Severe overt effects of parasitism indicate principle, be applicable to the family as a the degree to which the physiology of the whole. The same conclusions may not be host is affected. Mermithids cause either drawn, however, when considering the phys- gross degeneration or suppress the develop- iology of hosts that are so distantly related ment of a number of host tissues (9) which as orthoptera and diptera. In the ensuing in the normal insect fulfill vital metabolic discussion, therefore, effects of mermithid roles. Recent studies by Wfilker (49,50) infection on the two types of host are con- have significantly advanced our understand- sidered separately. ing of the cytological changes that occur in M. nigrescens in locusts: Jutsum and the imaginal discs and gonads of chiron- Goldsworthy (22) reported that relatively omids (Diptera) as a result of mermithid low infections (ca. three worms per host) of parasitism. Mermithids induce a greater M. nigrescens caused pronounced reduc- variety of developmental alterations of their tions in the levels of proteins within the hosts than do other entomogenous nema- blood and fat body of adult male Locusta todes. They prevent molting and, in some migratoria. instances, induce intersexuality (19,49), The physiological condition of the host gynandromorphism (19), or formation of influences the degree to which hemolymph intercastes (26). and tissue metabolite levels are affected by In 1927 Cobb et al. (4), intrigued by the infection. Electropherograms of hemolymph discovery that the sex of mermithids is de- taken from adult female Schistocerca termined by factors inherent in the level of gregaria 3 wk after infection with heavy parasitism, called for physiological studies doses of M. nigrescens (40-50 worms per on the host-parasite relationships. However, host) showed that almost all of the protein little attention was paid to such physiolog- fractions were depleted through parasitism ical considerations until recently. Two para- (12). These locusts had been infected soon sites have constituted the primary focus for after their imaginal molt, and the parasite physiological studies: Mermis nigrescens, prevented vitellogenesis. But in a study parasitic in and locusts, and where infection was delayed until vitello- R. culicivorax, parasitic in larval mos- genesis commenced, the mermithid did not quitoes. affect overall hemolymph protein levels (9). MERMITHIDS The initiation of infection was delayed ON HOST PHYSIOLOGY in these experiments, so the parasitized host commenced vitellogenesis, but the yolk was In 1940 Rempel (36) surmised from his- subsequently resorbed into the hemolymph. 268 Journal of Nematology, Volume 13, No. 3, July 1981 Such a resorption of vitellogenic proteins cause the fat body tissue would be deprived likely compensated [or the reduction in of amino acid precursors. Though the mech- hemolymph (protein) levels that would anisms involved are matters for speculation, otherwise have occurred. The mermithid there would appear to be sufficient evidence was found to cause a significant depletion to support the proposal that M. nigrescens of hemolymph proteins in fifth-instar S. acquires amino acids by inhibiting protein gregaria (infected as fourth instars) as the synthesis in addition to stimulating protein nematode embarked upon its rapid growth catabolism within tile insect's fat body (7). phase (7). Mermis nigrescens modified pro- Parasitism of L. migratoria or S. gregaria tein turnover in S. gregaria in a stepwise by M. nigrescens is characterized by a dras- fashion. The electrophoretic pattern of fat tic reduction in the total level of hemo- body soluble proteins was altered prior to lymt)h carbohydrates (9,22). In S. gregaria comparable alterations in hemolymph pro- tile depletive effect of parasitism on blood tein fractions (12). Compared to nonin- carbohydrates is caused by a dramatic re- fected control insects, proteins in fat bodies duction in hemolymph trehalose levels that of parasitized insects were found to be de- coincides with the nematode's rapid growth pleted at 2 wk after infection; correspond- phase (41). The mermithid is able to ing reductions in proteins were not recorded utilize glucose, but not trehalose, and must, from the hemolymph until 1 wk later. The therefore, modify host carbohydrate metab- 3d wk of infection represented a period of olism to favor the production of glucose. maintenance and reconstitution of fat body Gordon et al. (10) showed that the fat soluble proteins, but corresponding effects bodies of infected locusts contained approx- were not found for hemolymph proteins imately half the concentration of glycogen until the 4th wk of infection. as did those of controls, but fat body gly- Gordon and Webster (9) found that the cogen levels did not change during the 3rd overall level of amino acids in the hemo- wk of intection when the nematode grew lymph of adult female S. gregaria was not most rapidly and completed parasitic de- affected by mermithid parasitism, but the velopment. The parasitism caused a pro- content of amino acids within the fat body gressive and total depletion of glycogen was significantly reduced. In male hosts, phosphorylases in the host fat body (10), an Rutherford and Webster (41) recorded a effect which blocks glycogenolysis and ac- doubling of hemolymph amino nitrogen counts for the low, but stable, glycogen levels toward the end of the period of the levels. Fat body glycogen is not a nutrient parasite's development as a result of signif- source for the developing nematode and is icant increases in the levels of 11 amino depleted in parasitized locusts because its acids. The finding that the hemolymph of synthesis is impaired (41). Rutherford and infected insects was not depleted of amino Webster (41) found that trehalose was syn- acids led to the suggestion that the mer- thesized from injected glucose at a faster mithid obtains dietary protein nitrogen by rate by infected insects than by controls; stimulating protein catabolism by the host they suggested that tile nematode acquires fat body, thereby releasing amino acids into dietary glucose by stimulating the host fat the hemolymph and bringing about the ob- body to hydrolyze trehalose. served reductions in fat body proteins (12). The total level (22) and fatty acid com- Equally plausible hypotheses were advanced position (41) of hemolymph lipids in locusts by Rutherford and Webster (41), who sug- are unaffected by mermithid parasitism, but gested that amino acids could accumulate hemolymph levels of the sterols cholesterol in the hemolymph as a result of the host's and chloestanol appear to be increased (40). impaired excretory capacity, via excretion The mermithid reduces the host's capacity of amino acids by the nematode and/or by for lipid mobilization, thereby rendering secretion of an amino acid analog that in- prolonged flight inefficient or impossible hibited the host's amino acid transport sys- (22). tems. The last hypothesis would explain Excretion was not inhibited in lightly the impaired capacity of fat bodies of para- infected L. migratoria (22), but was im- sitized insects to synthesize proteins (7), be- paired in heavily infected S. gregaria (9). Mermithids--Physiological Relationships: Gordon 269 The hemolymph of infected hosts contained duce endocrinological disturbances in the more than five times the concentration of ]lost indirectly by disturbing the various uric acid found in controls, while the con- hormonal feedback systems that serve to centration of fecal uric acid was reduced to regulate metabolite levels. one-quarter of tile control level in para- Mermithids in larval diptera: Due to sitized insects (5). Accumulation of uric limitations imposed by the small size of the acid within the hemolymph likely results host in obtaining samples for analyses, physi- from tile host's reduced excretory capacity ological information on the effects of mer- and, possibly, the postulated increased ca- mithids on larval diptera is less readily ob- tabolism of fat body proteins, tained than for grasshoppers and locusts. Insects normally control such processes Studies have utilized "pooled" tissue or as protein metabolism, carbohydrate metab- hemolymph samples collected from large olism, lipid metabolism, and excretion via nmnbers of insects. their endocrine system (18). Since these Romanomermis culicivorax was found physiological processes are disrupted to deplete hemolymph proteins of Culex through mermithid parasitism, one might pipiens to one-sixth of the control levels reasonably expect to find an altered hor- (43). Using polyacrylamide gel electro- monal status in the parasitized host. How- phoresis, Gordon et al. (13) showed a sim- ever, there is no direct evidence of such ilar exhaustive effect of mermithid (Neo- endocrinological changes in locusts. Jutsum mesomermis flumenalis) parasitism on the and Goldsworthy (22) noted that sexual hemolymph proteins of the blackffies Pro- maturation of male locusts was not affected simuliurn mixtum/[uscum and Simulium by parasitism and reasoned that the activity venustum and, as in C. pipiens (43), the of the corpora allata must be unaffected reduction affected all detected protein frac- also. These authors provided evidence that tions. Wfilker (48), on tile other hand, re- M. nigrescens does not interfere with the corded no differences in proteins between release of adipokinetic hormone from the tile paper electropherograms of the blood host's corpora cardiaca. In response to in- of mermithid-infected and uninfected chi- jections of corpora cardiaca extracts, para- ronomid larvae. sitized insects mobilized lipids to a lesser Mermithid parasites of larval diptera degree than did controls, suggesting that probably resemble M. nigrescens in obtain- tile mermithid interferes with lipid mobil- ing dietary amino acids by stimulating the ization by reducing the response of the fat catabolism of proteins within the host fat body to tile hormone. Craig and Webster body. Total amino acid levels in the hemo- (7) found that ecdysone levels in locusts lymph of parasitized C. pipiens larvae were were unaffected by M. nigrescens parasitism, found to be the same as in controls when and they attributed the inhibition of molt- the hosts were reared at pH 4.5, but were ing in parasitized insects to depletion by inexplicably reduced compared to controls the nematode of precursors required by the when the rearing pH was raised to pH 7.4 host for protein and cuticle synthesis. How- (43). Studies on mermithid parasitized ever, the fact that the ovaries of parasitized chironomids (24) indicated that levels of locusts were unable to sequester vitellogenic several hemolymph amino acids were in- proteins available within the hemolymph creased substantially as a result of para- (12), a process also endocrinologically con- sitism, while only a few amino acids de- trolled (18), suggests endocrine dysfunction creased in concentration. In the simuliid in the host. Considering the degree to which S. venustum 14 of 32 ninhydrin positive sub- M. nigrescens affects a variety of physiolog- stances detected in the hemolymph were ical and developmental processes in the found to be increased by parasitism, 10 sub- host that are normally under endocrine stances decreased in concentration, and 8 control, it would be surprising, indeed, if substances were unaffected (13). The same tile host's endocrinology were not disrupted. study showed, by contrast, that mermithid The parasite may secrete substances that di- parasitism in the simuliid P. mixtum/ rectly affect host endocrine activity. More fuscum is characterized by a decrease in the likely, nematode feeding activity would in- majority of the hemolymph amino com- 270 Journal o] Nematology, Volume 13, No. 3, July 1981 pounds. Thus, as in locusts, mermithids do istry and microspectrophotometry, Condon not appear to cause a general drain on the and Gordon (6) found that N. flumenaIis hemolymph amino acid pool of larval disturbed the activity of the neuroendocrine diptera; in many instances, individual and system in S. venustum by significantly in- total amino acid levels were found to be creasing the nuclear DNA/RNA activity of unaffected or increased by parasitism. The the corpus allatum as well as the volume concept outlined above to explain the man- and amount of stored neurosecretory ma- ner by which the host fat body could pro- terial in the corpus cardiacum. Because the vide M. nigrescens with amino acids fits the mermithid was found not to induce notic- data available for these mermithids. In ad- able changes in the endocrinology of P. dition to the fat body, other tissues break mixtum/fuscum, it is unlikely that the down in parasitized Aedes aegypti (1) and endocrinological disturbances noted in para- undoubtedly provide nutriment for the de- sitized S. venustum have any bearing upon veloping nematode. the development of the nematode. In all Wiilker (49) noted that the hemolymph likelihood, the mermithid is not actively carbohydrate level was diminished in chi- manipulating the hormonal balance of its ronomids by mermithid parasitism. In para- host to modify its own microenvironment. sitized simuliids such reductions in carbo- The disturbances in S. venustum endocri- hydrate levels are caused by depletion of nology probably constitute one of several blood glucose but not trehalose (13). possible stressful symptoms of the para- Schmidt and Platzer (43) found that levels sitemia, induced by the nutritional dis- of total carbohydrates in the hemolymph of turbances caused by mermithid parasitism. C. pipiens were unchanged by R. culici- vorax parasitism. However, additional ex- HOST PHYSIOLOGY perimental details are needed to interpret ON MERMITHIDS this observation. Romanomermis culici- Little attention has been paid to the vorax was found to cause an almost com- reverse effect; i.e., the possibility that dif- plete exhaustion of Periodic Acid Schiff ferences in physiology within or among positive material within the fat body tissue hosts influence the parasitic development of .4. aegypti during the nematode's rapid of mermithids. This is unfortunate, since growth phase (1). Glycogen reserves were knowledge of such effects could provide shown to be similarly reduced in the fat important clues to the nature of the nema- bodies of mermithid parasitized simuliids tode's requirements for growth and develop- (6). In contrast to what we found to be the ment which would be applicable to in vitro case for M. nigrescens, mermithid parasites culture. of mosquitoes and simuliids cause almost From the host's standpoint, the most complete degeneration of host fat body tis- desirable response to an invading mennithid sue, All storage metabolites, including would be a successful defense mechanism. glycogen, within the fat body may, directly Mermithids may be either encapsulated by or indirectly, be utilized by the developing the host hemocytes (with or without nematode. melanin formation) or melanized by non- Gordon et al. (14) found that mermithid celhflar (humoral) components of the hemo- parasitism did not significantly affect either lymph. Present knowledge of immunity re- tile overall concentration of lipids or the sponses is limited to morphological con- relative proportions of the lipid fractions siderations, derived largely from light mi- in the hemolymph of A. aegypti. The mer- croscope and ultrastructural studies (32). mithid did, however, cause an increase in Studies have not been undertaken to ascer- the myristic acid:palmitic acid ratio in the tain the physiological events associated with free fatty acid faction of the host's hemo- the immunity mechanisms. Thus, we do lymph. not know, for example, what enables etedes Wiilker (49) recorded ultrastructural triseriatus to melanize R. culieivorax hu- abnormalities in the corpora allata of mer- morally, or CuIex territans to encapsulate mithid parasitized chironomid larvae and it (35), given the fact that this nematode pupae. Using a combination of histochem- will develop normally in more than 50 spe- Mermithids-Physiological Relationships: Gordon 271 cies of mosquitoes (32). In susceptible host incubation medium, and there was no sig- species some mermithids may be able to nificant difference in glucose uptake be- block the host's immunity mechanism(s) by tween anterior and posterior halves of inhibiting the melanization process. In chi- totally immersed worms (39). Ultrastruc- ronomids parasitism by mermithids re- rural evidence for tile transcuticular feeding tarded and weakened the capacity of the process was provided by Poinar (33,34) who hemolymph to melanize fungal parasites of showed that R. culicivorax can absorb fer- the flies (45). Such an impaired capacity of ritin across its outer cuticle. The cuticle of the host's blood for melanization may be a n~ermithids would appear on ultrastruc- consequence of the reduction in phenol- tural grounds to be well adapted for nutri- oxidase activity that occurs in the blood of ent absorption. The cuticle of the parasitic mermithid parasitized chironomids (25). stage(s) of Gastrornermis boophthorae is Certain host-parasite relationships offer very thin, contains iewer layers than subse- particularly intriguing lines of physiolog- quent free-living stages, and overlies a hy- ical inquiry. Unless the mermittlid Filip]evi- podermis replete with organelles indicative merrnis leipsandra first enters and partially of high metabolic activity; the hypodermal develops within a nerve ganglion of its surface is of a microvillous nature (2). coleopteran host before returning to the Cuticle ultrastructure may vary widely henlolymph, it is encapsulated (31). During among mermithids. Pores are apparently its period of development within the host present in the cuticle of R. culicivorax (33, nerve ganglion, the parasite persumably 34) but not in parasitic M. nigrescens (47) changes physiologically and/or induces or G. boophthorae (2). changes in host physiology to render itself There is some disagreement as to compatible upon returning to the hemo- whether mermithids secrete enzymes to lymph. The head region of A. triseriatus predigest host nutrients before absorbing provides a microenvironment where R. them. Rubtsov (37,38) theorized that the culicivorax can develop free from immunity cells of tile longitudinal cords secrete en- reactions, whereas development in other zymes to induce lysis of host fat body tissue, body regions is prevented by humoral and a variety of digestive enzymes were melanization (35). found in homogenates and exudates of Parasitic development may be modified mermithids from chironomids (44). according to the quality and/or quantity of It is highly unlikely that M. nigrescens the host's diet. In R. culicivorax the quan- secretes digestive enzymes. Radiotracer stud- tity of food available to the host affects the ies showed that the nematode is able only determination of sex (16,27). When para- to absorb simple molecules such as glucose sitized A. aegypti were fed on a diet low in (39) or amino acids (11) across its cuticle. quantity and/or protein content, parasitic Uptake of glucose and amino acids occurs development became asynchronous and by mediated transport systems, which are some of the nematodes from hosts infected not coupled to co-transport of sodium ions by more than one nematode emerged before and are not energy dependent (42). Our others had completed their development recent studies (unpublished) have shown (16). Thus, substantially fewer postparasites that palmitate, in R. culicivorax, is similarly were recovered from nutritionally deprived absorbed by a carrier system(s). Uptake of hosts. large molecules, such as ferritin (33,34), may be confined to species with porous PHYSIOLOGY OF MERMITHIDS cuticles. The nutrition of mermithids is con- Once absorbed, nutrients are transferred siderably modified from the basic nematode to the nematode's trophosome for metabolic pattern. There is general agreement that conversion. Nutrients may enter the tro- mermithids absorb nutrients from the host's phosome of G. boophthorae through cyto- hemolymph across their outer cuticle. The plasmic bridges that connect it to the hypo- ability of parasitic M. nigrescens to take up dermal cords and/or through the pseu- ~4C-glucose was not lessened when the head docoelom, since the outer surface of the of the nematode was positioned out o£ the trophosome is structurally well equipped 272 Journal of Nematology, Volume 13, No. 3, July 1981 for nutrient absorption (3). Batson (3) fore, the nematode is a facultative anaerobe. showed that the trophosome of G. booph- Comlitions oil the surface of the substratum, thorae is syncytial, without lumen, and con- however, would not be anoxic. It is not rains globules of high and low electron den- known how long a period of time is spent sity as well as reticulate granular inclusions. by the mermithid between emerging from Histochemical and analytical studies showed the host and penetrating the substratum. that lipids constitute the predominant stor- age metabolite in R. culicivorax, glycogen CONCLUSIONS the next most abundant, and proteins quan- During the past 10 yr a substantial titatively a minor storage product (21). amount of base-line data has been gathered Kaiser and Fachbach (23) demonstrated dif- to provide a general understanding of how ferences in the electrophoretic separation of the physiology of insects is altered by mer- trophosomal (soluble) proteins among sev- mithid parasitism. Details are still lacking, eral species of Hexamermis and suggested however. Thus, tile manner by which mer- that electropherograms of trophosomes mithids induce changes in the metabolic might be used as a taxonomic tool. Using processes of the host fat body is not known, both thin layer and gas chromatography, neither has the physiological basis of para- Gordon et al. (14) showed that the tro- sitic castration been elucidated. Why are phosomal lipids of R. culicivorax and N. the oocytes of parasitized insects unable to flumenalis comprised in order of pre- sequester proteins? The effect(s) of para- valence: triacylglycerols, phospholipids, free sitism on the host endocrine system is un- sterols, and sterol esters. The degree of un- clear and only in larval diptera have effects saturation of fatty acids within the triacyl- been demonstrated. But these small aquatic glycerol and sterol ester moieties was found insects are not ideal for such endocrinolog- to be greater for the boreally adapted N. ical studies. A larger terrestrial insect, such [tumenalis than for R. culicivorax (14). It as a locust, readily lends itself to endocrino- is possible that the higher proportion of logical manipulations (through micro- unsaturated fatty acids in N. flumenalis is surgery, injections, etc.), and such studies of adaptive significance in permitting main- would reveal whether the pronounced physi- tenance of physical state and consequent ological disturbances induced in the host enzymic activity at low temperatures. A emanate from, or cause, endocrinological variety of sterols were tentatively identified changes. from the free sterol and sterol ester frac- The effects of the host's physiology on tions of the trophosomes of R. culicivorax the developing nematode constitutes an and N. flumenalis (15); a C26 sterol pre- area that has been almost entirely neglected dominated in both nematodes but, surpris- and yet, from the standpoint of relevance to ingly, cholesterol was present in relatively in vitro culture, represents a highly reward- small amounts. ing field of study. What factor(s) within the Trophosomal storage products are re- host intluence or control mermithid devel- quired for energy metabolism, egg develop- opment? Does the parasite depend upon its ment, and, undoubtedly, a variety of other host purely for a supply of nutrients, or is processes as yet undetermined. In R. culici- its development controlled by "physiolog- vorax postparasites there is a functional fl- ical triggers" (e.g., hormones or specific oxidation pathway (17), the presence of metabolites) that would have to be supplied which correlates with the nematode's pre- in artificial culture media? ferred storage of triacylglycerols. To what More so than in most other nematodes, degree tile mermithid uses the fl-oxidation lipids constitute an extremely important pathway under natural conditions is specu- nutritional requirement for mermithids. lative, however, since details are lacking Studies ongoing in our laboratory are aimed concerning the oxygen tensions prevailing at elucidating the requirements of mer- within the nematode's microenvironment. mithids for specific lipid classes and the hnbriani and Platzer (20) suggest that con- manner by which such nutrients are ab- ditions within the substratum of the mer- sorbed. Such information could be of direct mithid's natural habitat are anoxic; there- benefit to researchers involved in in vitro Mermithids-Physiological Relationships: Gordon 273 culture attempts, especially when one con- hibition of molting of the , Schistocerca siders that commercially available synthetic gregaria, by the nematode Mermis nigrescens. Can. J. Zool. 52:1535-1539. culture media used for culturing nematodes 8. Finney, J. R. 1981. Mermithid nematodes: in are singularly deficient in lipids. vitro culture attempts. J. Nematol. 13:000-000. Aside from the abovementioned prob- 9. Gordon, R., and J. M. Webster. 1971. Mermis lems that are directly relevant to in vitro nigrescens: physiological relationship with its host, the adult desert locust Schistocerca gregaria. Exp. culture, mermithid nematodes present many Parasitol. 29:66-79. tantalizing problems for physiologists to 10. Gordon, R., J. M. Webster, and D. E. Mead. sort out. What physiological mechanisms 1971. Some effects of the nematode Mermis nigres- underly the host's defense mechanism and, cens upon carhohydrate metabolism in the fat body in F. leipsandra, what role does the insect's of its host, the desert locust Schistocerea gregaria. Can. J. Zool. 49:431-434. nervous system play in averting immunity 11. Gordon, R., and J. M. Webster. 1972. Nutri- responses? The fate of trophosomal storage tional requirements for protein synthesis during products, their incorporation into the de- parasitic development of the entomophilic nema- veloping eggs and utilization for other tode Mermis nigrescens. Parasitology 64:161-172. 12. Gordon, R., J. M. Webster, and T. G. Hislop. processes remains to be investigated. While 1973. Mermithid parasitism, protein turnover and the oxygen content of the environment of vitellogenesis in the desert locust, Schistocerca an aquatic mermithid such as R. culicivorax gregaria Forskol. Cmnp. Biochem. Physiol. 46B:575- is in some doubt, a terrestrial mermithid 593. such as M. nigrescens burrows into the soil 13. Gordon, R., W. J. Condon, W. J. Edgar, and S. J. Bahie, 1978. Effects of mermithid parasitism on up to a depth of 45 cm-hardly an aerobic the hemolymph composition of the larval blackflies situation. If, as seems likely, the nematode Prosimulium mixtum/fuscum and Simulium venu- engages in facultative anaerobiosis, it prob- stum. Parasitology 77:367-374. ably does not use the phosphoenolpyruvate 14. Gordon, R., J. R. Finney, W. J. Condon, and T. N. Rusted. 1979. Lipids in the storage organs of carboxykinase pathway characteristic of three mermithid nematodes and in the hemolymph many other facultative anaerobes (30). How of their hosts. Comp. Biochem. Physiol. 64B:369-374. does it metabolize in such an environment, 15. Gordon, R., W. J. Condon, and J. M. Squires. in which it may live for up to 3 yr? 1980. Sterols in the trophosomes of the mermithid nematodes Neomesomermis flumenalis and Romano- As in any rewarding field of research, mermis culicivorax relative to sterols in the host there appear to be more questions than hemolymph. J. Parasitol. 66:585-590. answers ! 16. Gordon, R., J. M. Squires, S. J. Babie, and I. R. Burford. 1981. 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