The seven larval instars of Prosimulium mixtum Syme and Davies and P. fuscum Syme and Davies (Diptera: Simuliidae)

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Abstract
Larval Prosimulium mixtum/fuscum pass through seven instars dearly segregated by postgenal lengths (P < 0.001). Total body lengths and number of cephalic fan rays differ significantly between instars one through six (P < 0.001), but not between sixth and seventh instars. However, the last two instars are separated by degree of development of the pupal respiratory organ. Scanning electron microscope examination revealed variations in the number of rows of hooks and hooks per row in the posterior circlet, hypostomial teeth, and in mouthpart structure. However, these three characters do not possess distinctly defined states and therefore are not diagnostic in separating all instars. Structural differences in mouthparts of the instars appear to be adaptations for different food-collecting habits, a phenomenon seen in certain species in other simuliid genera. A key to the seven larval instars of P. mixtum/fuscum is also presented.

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