Tsetse Fly

A tsetse fly (pronounced /ˈtsɛtsi/ or /ˈsiːtsi/; from the Setswana word tsetse, meaning "fly destructive to cattle") is any of approximately 31 species and subspecies of large, obligately blood-feeding insects comprising the sole genus Glossina within the family Glossinidae (superfamily Hippoboscoidea, order Diptera, section Schizophora). Although Oligocene compression fossils (~34 Ma) from the Florissant Formation of Colorado prove that Glossina once inhabited North America, extant tsetse populations are strictly endemic to the sub-Saharan African "tsetse belt"—an area of roughly 8.5 to 10 million km² spanning 38 countries between latitudes 14°N (the southern margin of the Sahara and Sahel) and 20°S (the Kalahari and Namib deserts). Unlike mosquitoes (Anopheles, Aedes, Culex) and black flies, in which only adult females take blood meals and lay hundreds of eggs in water, both male and female tsetse flies are obligate hematophages that feed exclusively on vertebrate blood throughout their adult lives, and females reproduce via adenotrophic viviparity—gestating a single larva at a time inside the maternal uterus on a mammary-like "milk gland" secretion (Benoit et al., 2015). Biologically and economically, Glossina species are the primary cyclical vectors of flagellated protozoan parasites of the genus Trypanosoma, transmitting human African trypanosomiasis (HAT, or sleeping sickness) and animal African trypanosomiasis (AAT, or nagana), which historically excluded cattle husbandry and draft animal traction across humid tropical Africa (Jordan, 1986; Aksoy et al., 2014).

Within the calyptrate Diptera, family Glossinidae belongs to the pupiparous superfamily Hippoboscoidea alongside louse flies (Hippoboscidae) and bat flies (Streblidae and Nycteribiidae), all of which share matrotrophic intrauterine larval development. Adult tsetse flies measure 6 to 16 mm in body length, exhibit yellowish-brown to dark gray-black integumental coloration, and are immediately distinguishable from other biting flies (such as stable flies, Stomoxys calcitrans, and horseflies, Tabanidae) by four anatomical synapomorphies:

1. Scissor-Blade Wing Folding: At rest, a tsetse fly folds its two mesothoracic wings completely flat one atop the other over the dorsal abdomen like the closed blades of a pair of scissors, extending slightly beyond the abdominal tip.
2. The Cleaver ("Hatchet") Discal Medial Cell: In the center of the wing venation, the discal medial cell (dm cell) is uniquely shaped like a butcher's meat cleaver or hatchet.
3. Branched Plumose Arista: The third antennal segment (postpedicel), which houses dense olfactory sensilla inside recessed sensory pits, bears a dorsal arista fringed along its upper margin by a row of hairs that are themselves secondarily branched (feather-like).
4. Forward-Projecting Haustellum Proboscis: The piercing–sucking proboscis (formed by the sclerotized labium, labrum-epipharynx, and salivary-duct-carrying hypopharynx) projects straight forward in front of the head, ensheathed at rest between two sensory maxillary palps of equal length; during feeding, a bulbous muscular expansion at the base (thecal bulb) everts microscopic rasping prestomal teeth at the labellar tip to lacerate dermal capillaries.

Taxonomists divide the ~31 recognized taxa of Glossina into three monophyletic subgenera (species groups) that occupy distinct ecological biomes and transmit different Trypanosoma species:

1. Adenotrophic Viviparity and K-Selected Demography

Whereas an Anopheles mosquito lays 100 to 200 eggs per gonotrophic cycle, a female Glossina matures only a single oocyte at a time, alternating sequentially between the two polytrophic ovarioles of her right and left ovaries every 9 to 10 days:

• After insemination—during which the male transfers a large chitinous spermatophore into the uterus so that spermatozoa migrate into the paired spermatheca for life-long storage—a single fertilized egg ovulates into the muscular uterus.
• The embryo hatches in utero into a first-instar larva (L1), which feeds through its buccal cavity on nutritious secretions pumped from a branched tubular accessory gland called the milk gland. Regulated by juvenile hormone and insulin-like signaling, tsetse milk consists of ~50% lipids (emulsified triacylglycerols) and ~50% proteins—dominated by Milk Gland Protein 1 (MGP1), a lipocalin functional ortholog of mammalian marsupial lactation proteins, alongside transferrin and acid sphingomyelinase (Benoit et al., 2015).
• The larva molts through the second (L2) and third (L3) instars inside the uterus while respiring through a pair of posterior spiracular lobes pressed against the maternal vulva. At day 9–10, the mother deposits (larviposits) a fully grown third-instar larva (nearly equal in wet mass to the mother herself) onto shaded soil. Within 1 to 2 hours, the larva burrows 2–5 cm underground without ever feeding externally, contracts into a barrel-shaped puparium with two prominent posterior polypneustic lobes, tans its insect cuticle via catecholamine sclerotization, and completes holometabolism pupal development over ~30 days. Because a female produces at most 8 to 12 offspring over a 3-month lifespan, tsetse populations possess a very low intrinsic rate of natural increase (r_m), making them uniquely vulnerable to moderate sustained increases in adult daily mortality (>3%–4% day⁻¹).

2. Proline-Fueled Flight Bioenergetics and Aquaporin Diuresis

Because both sexes feed strictly on vertebrate blood (and never ingest floral nectar carbohydrates), Glossina has evolved a specialized amino-acid catabolic pathway to power insect flight. The fat body converts blood-meal alanine and lipids into the amino acid L-proline, which circulates in the hemolymph at ~50–150 mM and is oxidized inside flight-muscle mitochondria via proline dehydrogenase into glutamate and α-ketoglutarate (returning alanine to the fat body in a closed proline–alanine shuttle). Simultaneously, after imbibing a blood meal up to twice its unfed body weight, the fly excretes ~40%–50% of the water and excess NaCl within 30 minutes via serotonin- and diuretic-hormone-stimulated Malpighian tubules expressing up to 10 aquaporin water-channel genes (International Glossina Genome Initiative, 2014).

3. The Tripartite Bacterial Symbiosis (Wigglesworthia, Sodalis, and Wolbachia)

Because vertebrate blood is deficient in B-complex vitamins, tsetse flies harbor an obligate intracellular γ-proteobacterial mutualist, Wigglesworthia glossinidia, housed inside specialized polyploid midgut epithelial cells (bacteriocytes) that form a U-shaped organ called the bacteriome (as well as extracellularly inside the lumen of the female milk gland for vertical transmission to the developing intrauterine larva). Sequencing of the reduced 700-kb Wigglesworthia genome by Serap Aksoy and colleagues (2002) proved that the bacterium retains complete biosynthetic pathways for thiamine (vitamin B₁), riboflavin (B₂), nicotinamide (B₃), pantothenate (B₅), pyridoxine (B₆), biotin (B₇), and folate (B₉). Antibiotic elimination of Wigglesworthia renders female tsetse flies completely sterile (aposymbiotic abortion) and compromises synthesis of the midgut peritrophic matrix (Aksoy et al., 2014).

Alongside Wigglesworthia, natural tsetse populations frequently carry two additional maternally transmitted bacteria:

• Sodalis glossinidius: A facultative, microaerophilic γ-proteobacterium residing across midgut, muscle, fat body, and salivary gland tissues, which is used as a genetic vehicle for paratransgenic expression of anti-trypanosomal nanobodies.
• Wolbachia pipientis: An α-proteobacterial endosymbiont infecting the ovaries and testes that induces cytoplasmic incompatibility (CI) when infected males mate with uninfected females.

Unlike horseflies (Tabanidae), which transmit trypanosomes only mechanically on contaminated mouthparts, tsetse flies serve as the obligate biological hosts for the 18- to 35-day extrinsic developmental cycle of Salivarian trypanosomes:

1. Midgut Establishment and the Peritrophic Matrix Barrier: When a tsetse fly ingests mammalian blood containing short stumpy bloodstream-form Trypanosoma brucei, the parasites differentiate within the midgut lumen into proliferative procyclic trypomastigotes (switching surface coats from variant surface glycoprotein, VSG, to procyclin). To survive, procyclics must traverse the sleeve-like Type II peritrophic matrix continuously secreted by the proventricular cardia, migrate anteriorly within the ectoperitrophic space back to the proventriculus, and re-penetrate the peritrophic matrix into the foregut. Due to midgut immune antimicrobial peptides (attacin, cecropin), reactive oxygen species, and peritrophic matrix thickness, >90% of tsetse flies clear the midgut infection, resulting in mature salivary gland infection rates in wild flies of <0.1% to 1%.
2. Salivary Gland Metacyclogenesis: Survivors migrate up the hypopharynx into the salivary glands (in Trypanosoma brucei; whereas T. congolense matures in the proboscis and T. vivax develops entirely within the proboscis), attach as epimastigotes to the salivary microvilli, undergo meiotic genetic exchange, and differentiate into free-swimming, non-dividing metacyclic trypomastigotes that re-acquire a protective VSG coat ready for inoculation into a vertebrate host during the next blood meal.

Visual–Olfactory Trapping ("Tiny Targets") and Sterile Insect Eradication

Because tsetse larvae develop underground and only one offspring is produced every 10 days, integrated pest management against Glossina targets the adult fly with four low-impact methods:

• Phthalogen-Blue/Black Visual Traps and Odor Baits: Savanna and riverine tsetse are strongly attracted by compound-eye ommatidia to royal phthalogen blue (reflectance peak ~460 nm) paired with ultraviolet-absorbing black cloth targets impregnated with deltamethrin. For savanna species (G. morsitans and G. pallidipes), baiting traps with synthetic host kairomones—acetone, 1-octen-3-ol, 4-methylphenol, and 3-n-propylphenol (mimicking ox breath and buffalo urine)—increases catch rates by 5- to 20-fold, whereas small 25 × 50 cm insecticide-treated "tiny targets" staked along riverbanks reduce riverine G. fuscipes and G. palpalis densities by >90% and have helped lower reported human sleeping sickness cases below 1,000 annually (Tirados et al., 2015).
• The Sterile Insect Technique (SIT) on Unguja Island (Zanzibar): Between 1994 and 1997, the International Atomic Energy Agency (IAEA) and Tanzanian authorities combined pour-on pyrethroids on livestock with weekly aerial releases of >8.5 million gamma-irradiated sterile male Glossina austeni across the 1,650 km² island of Unguja (Zanzibar). Because wild virgin females typically mate only once and store sperm in the spermatheca, mating with sterile males permanently arrested embryogenesis, achieving complete, verified island-wide eradication of G. austeni and cattle nagana (Vreysen et al., 2000).

1. Jordan, A. M. (1986). Trypanosomiasis Control and African Rural Development. Longman. https://doi.org/10.1016/0169-4758(86)90166-390166-3)
2. Vreysen, M. J. B., Saleh, K. M., Ali, M. Y., Abdulla, A. M., Zhu, Z. R., Juma, K. G., Dyck, V. A., Msangi, A. R., Mkonyi, P. A., & Feldmann, H. U. (2000). Glossina austeni (Diptera: Glossinidae) eradicated on the island of Unguja, Zanzibar, using the sterile insect technique. Journal of Economic Entomology, 93(1), 123–135. https://doi.org/10.1603/0022-0493-93.1.123
3. Aksoy, S., Weiss, B., & Attardo, G. (2014). Trypanosome transmission dynamics in tsetse. Current Opinion in Insect Science, 3, 43–49. https://doi.org/10.1016/j.cois.2014.07.003
4. International Glossina Genome Initiative. (2014). Genome sequence of the tsetse fly (Glossina morsitans): Vector of African trypanosomiasis. Science, 344(6182), 380–386. https://doi.org/10.1126/science.1249656
5. Benoit, J. B., Attardo, G. M., Baumann, A. A., Michalkova, V., & Aksoy, S. (2015). Adenotrophic viviparity in tsetse flies: Potential for population control and as an insect model for lactation. Annual Review of Entomology, 60(1), 351–371. https://doi.org/10.1146/annurev-ento-010814-020834