UC Davis

Insect vision is renowned for being remarkably poor in terms of resolution, a situation worsened by low levels of light. In dim light, the heightened temporal resolution of insect eyes is also reduced, particularly in small species. Adult male Strepsiptera have among the fewest ommatidia (i.e., ocular facets) of any flying insect (10–150). The number is most diminished in species active in low light, which have fewer but larger ommatidia, and in smaller species. All well-characterized insect compound eyes perceive a single point per ommatidium, but given the extent of the reduction and other physiological traits, is not clear whether each strepsipteran ommatidium only perceives a single point, or is ever able to resolve additional detail in accord with the large number of photoreceptors in each respective “retinula” (little retina)—rather than 8, as is typical of other insect ommatidia, there are 50 to perhaps as many as 100 in diurnal species, such as Stylops pacifica, Xenos vesparum, and X. peckii. This is especially odd since light is plentiful during the day. (The number of photoreceptors per ommatidium is unknown for most Strepsiptera, including any crepuscular or nocturnal species.) The body of this dissertation consists of three chapters that help clarify and extend what is known about vision in Strepsiptera. Chapters 1 and 2 have already been published; chapter 3 is expected to be.Chapter 1 highlights my collaboration with the Buschbeck lab (University of Cincinnati) to determine if Strepsiptera have the capacity to see in color. Xenos peckii, a diurnal paper wasp-infecting species, was only found to express two opsins, rather than the three typically found in insect compound eyes. We determined that the tow opsins maximally absorb in the long wavelength band (539 nm; green), and the UV band (346 nm). Strepsiptera lack ocelli, which when present, often express a separate UV and green opsin. Thus, I posit that between the compound eyes and ocelli, all flying insects have a short and a long wavelength band opsin, on account of their usefulness in quickly distinguishing ground from sky. Due in large part to their enormous eyes, we assumed adult male Strepsiptera must use vision to finally identify calling females (i.e., emitting sex pheromone) after having tracked them countless meters via olfaction.Chapter 2 describes using a light trap for field collection of live Elenchus koebelei, a Strepsiptera active at dawn. Live collection was conducted to provide pristine specimens for future ocular structural comparison and analysis via electron microscopy. Prolonged collection demonstrated that reliable populations of Strepsiptera are available from hosts that inhabit fixed locations that only require single determination, unlike the more frequently casually encountered paper wasp-infecting species, whose individual nest sites are relocated each mating season. It furthermore provided the potential for direct interaction with flying specimens. In one such instance, I unsuccessfully attempted to aspirate an airborne E. koebelei (≈ 0.8 mm), which made it clear that whatever visual and other sensory strategies Strepsiptera employ enable them to deftly avoid direct encounters with potential threats.Chapter 3 centers on my analysis of what I believe to be the first-ever photographs of flying Strepsiptera, the nocturnal species, Triozocera texana, which I took in the field. From these and historical descriptions of various flying Strepsiptera, I learned that the often incongruous accounts of strepsipteran flight (e.g., that they fly quickly, that they fly slowly, that they are sporadic, that they are graceful, that they are excessively active, that they hover in place, etc.) are nearly all shared with sex pheromone plume-following moths. The only exceptions were the erratic flight of Strepsiptera confined to tight spaces, and the patrolling undertaken by adult males of species infecting diurnal solitary bees, the hosts of which overwinter in stable nesting areas. In these species, the typical application of olfaction at distance and sight in the vicinity of sex pheromone-emitting females is apparently inverted, although final determinations are still made chemotactically. Such males are larger and diurnal, and thus have bigger eyes with more, but smaller, ommatidia than other Strepsiptera (up from the 20–65 found in many others to ≈150 in the families Xenidae and Stylopidae). From a description of an incompletely eclosed flying Hylechrus rubi (family Halictophagidae) dragging its dead host, I calculated its flight muscle to total mass ratio (FMR) to be at least 44%, which should be typical for Strepsiptera. That value just surpasses the most flight muscle-endowed sphinx moths (0.436) and is only known to be exceeded by the upper echelon of odonate FMRs (damsel- and dragonflies at 0.473 and 0.560–0.63, respectively). However, unlike those species, adult male Strepsiptera do not feed and die on the same day they eclose. Although their flightworthy wings appear to be teneral, Strepsiptera are clearly not weak fliers. Moreover, strepsipteran flight patterns and mate pursuit show no sign of being disadvantaged by their potentially poor eyesight, which does not ever appear to require super-ommatidial resolution, except perhaps when approaching an agile and agitated host bearing a calling female. Even then, however, it is not the female that is ever identified visually, but instead her host, with a taxon-dependent level of specificity.The three mandatory chapters are followed by a coda that examines the possibility of Strepsiptera having a super-ommatidial resolution visual mode, but without the benefit of a comparative analysis of the ultrastructure of the eyes of a diurnal, a crepuscular, and a nocturnal strepsipteran species, as originally intended. It also discusses what additional information is required to fully characterize strepsipteran vision and how that could be achieved.