Breaking the rule: Five larval instars in the podonomine midge Trichotanypus alaskensis Brundin from Barrow, Alaska

Five larval instars in Trichotanypus alaskensis

Submitted: 20 January 2018
Accepted: 26 April 2018
Published: 6 June 2018
Abstract Views: 846
PDF: 354
Publisher's note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.


Except for one unconfirmed case, chironomid larvae have been reported to pass through four 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 in Chironomidae, 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.



PlumX Metrics


Download data is not yet available.


Supporting Agencies

U.S. Arctic Landscape Conservation Cooperative, U.S. National Fish and Wildlife Foundation, Environmental and Conservation Sciences Program at NDSU
Alec R. Lackmann, North Dakota State University, USA
Biological Sciences, Ph.D. Candidate
Malcolm G. Butler, North Dakota State University

Biological Sciences


Dr. Malcolm G. Butler

How to Cite

Lackmann, Alec R., and Malcolm G. Butler. 2018. “Breaking the Rule: Five Larval Instars in the Podonomine Midge Trichotanypus Alaskensis Brundin from Barrow, Alaska: Five Larval Instars in Trichotanypus Alaskensis”. Journal of Limnology 77 (s1).

List of Cited By :

Crossref logo