Challenging limits: ultrastructure and size-related functional constraints of the compound eye of Stigmella microtheriella (Lepidoptera: Nepticulidae).

With a body length of only 2 mm, the nepticulid Stigmella microtheriella (Stainton, 1854) is one of the smallest moths known to date. We investigated the optical design of its lemon-shaped compound eyes, which measure 83.60 μm in anterior-posterior and 119.77 μm in dorso-ventral direction. The eyes consist of about 123 facets, each of the latter just 9.9 μm in diameter. Transmission electron microscopy reveals an optical design with features intermediate between apposition and superposition optics similar to that known from two other small species of moths (one Nepticulid and one Gracillarid). Size-related evolutionary adaptations of the ommatidial organization include (1) the involvement of only five rhabdomeres in the formation of the distal rhabdom (2) the complete absence of a rhabdomere of the eighth (= basal) retinula cell, (3) the "hourglass" shape of the rhabdom with a characteristic narrow waist separating distal from proximal portion, and (4) the reduction to one single layer of tracheoles as an adaptation to the overall restricted space available in this minute eye.