J. Limnol., 2018; 77(s1): 104-112 DOI: 10.4081/jlimnol.2018.1805 This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0). INTRODUCTION Hemogoblin (Hb) happens to be the most studied of all proteins. Occurrence of hemoglobins (Hbs) across living kingdoms, its diversity amongst various taxa, and its complex developmental regulation makes Hbs a fascinating family of proteins and genes (Hardison, 1998; Vinogradov et al., 2005). Hbs are ubiquitous among all the vertebrates known till date with little variation in the quaternary structure. On the other hand, extracellular Hbs of invertebrates are sporadic and whenever present, the Hbs exhibit large scale structural heterogeneity (Vinogradov, 1985; Weber and Vinogradov, 2001) making these globins interesting and intriguing from the physiological, ecological and evolutionary context. Comparative information on invertebrate globins is very less compared to wealth of literature available for vertebrate globins. In the past, red body fluid in the invertebrates was described as cruorine, erythrocruorin and also as hemerythrin or hemoglobin depending on the time of publication and organisms (Terwilliger, 1980). The body fluid of many invertebrates is characterized by presence of respiratory pigments other than Hb, which are reviewed by Weber and Vinogradov (2001). Among insects, Hbs have been fully characterized till date only from Chironomus thummi thummi, Drosophila melanogaster and the larvae of the parasite botfly Gasterophilus intestinalis. Amongst these, Chironomus Hbs are extracellular, freely circulating in the hemolymph, while the Hbs of Drosophila and Gasterophilus are intracellular (Osmulski and Leyko, 1986; Pesce et al., 2005; Burmester et al., 2006). Chironomid non-biting midges comprise one of the largest insect families (Insecta: Order Diptera) inhabiting diverse freshwater ecosystem across the world. Lifecycle of the non-biting midges consists of aquatic egg, larval, pupal and terrestrial adult stages. Aquatic larval stage is the longest and these larvae are conspicuous in any freshwater ecosystem due to the presence of high concentration of Hb which renders them a bright coloration and hence the genesis of its popular name as ‘blood worms’ (Oliver 1971; Pinder 1986; Armitage et al., 1995; Ferrington, 2008). Chironomid midge hemoglobin (Ch-Hb) attracted attention of molecular evolutionary biologists with Antoine and Niessing’s publication (1984) Extracellular hemoglobin and environmental stress tolerance in Chironomus larvae Bimalendu B. NATH * Stress Biology Research Laboratory, Department of Zoology, Savitribai Phule Pune University, Pune 411007, India *Corresponding author: bbnath@gmail.com ABSTRACT Hemoglobin (Hb) is one of the most common conserved molecules found in organisms belonging to all major kingdoms of life. Chironomid midge larvae are unique among the invertebrates being the only free-living group of organisms possessing extracellular hemoglobins (Hbs) in monomeric and dimeric forms floating in their hemolymph. Remarkable ability of individual species of chironomid midges to tolerate wide range of environmental stressors prevailing in their respective ecosystems has enabled chironomids to sustain as one of the most widely distributed insects in the world. Ability of different Chironomus spp to thrive under extreme hypoxic conditions as well as in the presence of chemical pollution made chironomid midges an efficient model system to assess the effect of environmental stress in different freshwater ecosystems. The modulation of Hb concentration has been found to be a function of different type of abiotic stressors and thus Hb of chironomid midges (Ch-Hb) has attracted the attention as a potential biomarker for environmental biomonitoring. Large body of literature on Ch-Hb accumulated mainly during sixties to eighties of twentieth century has enriched our understanding of its physiological, developmental and biochemical features. Empirical as well as in-silico studies carried out in recent years provided insights into many structure-function relationships of Ch- Hb. However, for the last few decades, majority of the studies were carried out to project and validate Ch-Hb as potential bioindicator for evaluating toxicants, chemical pollutants and environmental gradients of dissolved oxygen concentration. In this review, historical development of the subject has been compiled with notes on future implications of contemporary studies on Ch-Hb. Key words: Chironomus; chironomid midge; extracellular hemoglobin; stress tolerance; hypoxia; biomarker; biomonitoring freshwater ecosystem. Received: March 2018. Accepted: October 2018. This paper was presented at the 20 th International Symposium on Chironomidae, Trento, Italy, 2-8 July 2017. Session: Autecology and Physiology. Non-commercial use only