Springer Nature is making SARS-CoV-2 and COVID-19 research free. View research | View latest news | Sign up for updates

Inequalities in body size among mermithid nematodes parasitizing earwigs


Variation among body sizes of adult parasitic worms determines the relative genetic contribution of individuals to the next generation as it affects the effective parasite population size. Here, we investigate inequalities in body size and how they are affected by intensity of infection in Mermis nigrescens (Mermithidae: Nematoda) parasitizing the European earwig Forficula auricularia in New Zealand. Among a population of pre-adult worms prior to their emergence from the host, we observed only modest inequalities in body length; however, among worms sharing the same individual host, inequalities in body sizes decreased with increasing intensity of infection. Thus, the more worms occurred in a host, the more the second-longest, third-longest and even fourth-longest worms approached the longest worm in body length. This pattern, also known from another mermithid species, suggests that worms sharing the same host may have infected it roughly simultaneously, when the host encountered a clump of eggs in the environment. Thus, the life history and mode of infection of the parasite may explain the modest inequalities in the sizes achieved by pre-adult worms, which are lower than those reported for endoparasitic helminths of vertebrates.

This is a preview of subscription content, log in to check access.

Fig. 1
Fig. 2
Fig. 3
Fig. 4


  1. Baylis HA (1944) Observations on the nematode Mermis nigrescens and related species. Parasitology 36:122–132

  2. Beck MA, Goater CP, Colwell DD (2015) Comparative recruitment, morphology and reproduction of a generalist trematode, Dicrocoelium dendriticum, in three species of host. Parasitology 142:1297–1305

  3. Criscione CD, Blouin MS (2005) Effective sizes of macroparasite populations: a conceptual model. Trend Parasitol 21:212–217

  4. Dobson AP (1986) Inequalities in the individual reproductive success of parasites. Parasitology 92:675–682

  5. Hanelt B (2009) An anomaly against a current paradigm: extremely low rates of individual fecundity variability of the Gordian worm (Nematomorpha: Gordiida). Parasitology 136:211–218

  6. Mössinger J, Wenk P (1986) Fecundity of Litomosoides carinii (Nematoda: Filarioidea) in vivo and in vitro. Z Parasitenkunde 72:121–131

  7. Poulin R (1996) The evolution of life history strategies in parasitic animals. Adv Parasitol 37:107–134

  8. Poulin R (2007) Evolutionary ecology of parasites, 2nd edn. Princeton University Press

  9. Poulin R, Latham ADM (2002) Inequalities in size and intensity-dependent growth in a mermithid nematode parasitic in beach hoppers. J Helminthol 76:65–70

  10. Poulin R, Latham ADM (2003) Effects of initial (larval) size and host body temperature on growth in trematodes. Can J Zool 81:574–581

  11. Presswell B, Evans S, Poulin R, Jorge F (2015) Morphological and molecular characterization of Mermis nigrescens Dujardin, 1842 (Nematoda: Mermithidae) parasitizing the introduced European earwig (Dermaptera: Forficulidae) in New Zealand. J Helminthol 89:267–276

  12. Saldanha I, Leung TLF, Poulin R (2009) Causes of intraspecific variation in body size among trematode metacercariae. J Helminthol 83:289–293

  13. Shostak AW, Dick TA (1987) Individual variability in reproductive success of Triaenophorus crassus Forel (Cestoda: Pseudophyllidea), with comments on use of the Lorenz curve and Gini coefficient. Can J Zool 65:2878–2885

  14. Sinniah B, Subramaniam K (1991) Factors influencing the egg production of Ascaris lumbricoides: relationship to weight, length and diameter of worms. J Helminthol 65:141–147

  15. Szalai AJ, Dick TA (1989) Differences in numbers and inequalities in mass and fecundity during the egg-producing period for Raphidascaris acus (Nematoda: Anisakidae). Parasitology 98:489–495

  16. Walker M, Hall A, Basánez M-G (2010) Trickle or clumped infection process? An analysis of aggregation in the weights of the parasitic roundworm of humans, Ascaris lumbricoides. Int J Parasitol 40:1373–1380

  17. Weiner J, Solbrig OT (1984) The meaning and measurement of size hierarchies in plant populations. Oecologia 61:334–336

Download references


We thank Steven Evans for assistance with earwig collection. FM was supported by a postdoctoral fellowship from Fondation Fyssen (France).

Author information

Correspondence to Robert Poulin.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Maure, F., Poulin, R. Inequalities in body size among mermithid nematodes parasitizing earwigs. Parasitol Res 115, 4471–4475 (2016).

Download citation


  • Body size
  • Competition
  • Gini coefficient
  • Mermithidae
  • Mermis nigrescens
  • Forficula auricularia