Breaking the rule: Five larval instars in the podonomine midge trichotanypus alaskensis brundin from Barrow, Alaska

Alec R. Lackmann, Malcolm G. Butler

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5 Scopus citations


Except for one unconfirmed case, chironomid larvae have been reported to pass throughlfour larval instars between egg and pupal stages. We have observed a fifth larval instar to be a standard life-cycle feature of the podonomine Trichotanypus alaskensis Brundin 1966 in tundra ponds on the Arctic Coastal Plain near Barrow, Alaska. T. alaskensis has a one-year life cycle in these arctic ponds. Adults emerge in June ~2-3 weeks after pond thaw, then mate and oviposit; most newly-hatched larvae reach instar IV by October when pond sediments freeze. Overwintering larvae complete instar IV within a few days of thaw, then molt again to a fifth larval instar. Imaginal discs, normally seen only during instar IV inChironomidae, develop across both instars IV & V prior to pupation and adult emergence. While monitoring larval development post-thaw in 2014, we noticed freshly-molted T. alaskensis larval exuviae a week or more prior to any pupation by that species. In 2015-16 we reared overwintering instar IV larvae from single pond sources, individually with daily monitoring, through molts to instar V, pupa, and adult. Some overwintering instar II and III larvae were reared as well, but were few in number. During 2016 we also reared T. alaskensis progeny (from eggs) through instar II, thus documenting head capsule size ranges for all five instars in a single pond’s population. Without individual rearings, the fifth larval instar was not readily apparent for two reasons: 1) The molt itself occurs immediately after thaw and is so synchronous it is difficult to discern in daily field samples. 2) The head capsule size increment between instars IV-V is much lower than the ratio predicted by the Brooks-Dyar Rule. Up through instar IV, the Brooks-Dyar ratio for T. alaskensis ranged 1.30-1.61, but during the IV-V molt head capsule dimensions (sexes pooled) increased by a ratio of 1.09 – comparable to the magnitude of sexual dimorphism in head capsule size within each of the final two larval instars. Individual rearings coupled with 2014-2016 field surveys in nine other ponds suggest that five larval instars is an obligatory trait of this species at this location. As this is the first confirmed case of five larval instars in a chironomid, the phylogenetic uniqueness of this trait needs further investigation.

Original languageEnglish (US)
Pages (from-to)31-39
Number of pages9
JournalJournal of Limnology
Issue numberSpecial Issue 1
StatePublished - Nov 13 2018
Externally publishedYes

Bibliographical note

Funding Information:
This work was completed at North Dakota State University (NDSU), USA as part of a doctoral dissertation byARL. The North Slope Borough and the Ukpeagvik Inupiat Corporation permitted access to the field site. We thank Ewelina Bielak-Lackmann, Daniel C. McEwen, and Kevin M. Cortes for assistance in the field and lab, and Shane D. Braegelman for helpful discussion and advice. We gratefully acknowledge financial support from the U.S. Arctic Landscape Conservation Cooperative, the U.S. National Fish and Wildlife Foundation, and the Environmental and Conservation Sciences Program at NDSU.

Publisher Copyright:
© 2018, Page Press Publications. All rights reserved.


  • Arctic
  • Brooks-dyar rule
  • Chironomidae
  • Larval exuviae
  • Podonominae


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