The Strepsiptera, or twisted wing insects, are an endoparasitic insect order in which the females, in all but the most basal family, never leave their host. The males on the other hand do emerge, but they only live for a few hours, during which time they have to find a female for mating (Fig.1). Strepsiptera are most frequently mentioned because the systematic position of this insect order remains still unresolved. In our lab we so far have primarily focused on their unusual eyes. Adult males are characterized by a visual system that uses design principles from compound as well as simple eyes. The lenses of this eye are unusually large and focus images onto extended retinae. The light-gathering ability of the lens is sufficient to resolve multiple points of an image in each optical unit, . which functions as independent image-forming eye that contributes an inverted partial image. Each partial image is re-inverted by optic chiasmata between the retinae and the lamina. This way upright images of neighboring units are assembled to a more or less coherent image at the level of the first visual neuropil, the lamina (Fig. 2).
As in most insects, the optic lobes contain a lamina, medulla and lobula. However in contrast to other insects there is no clear organization into cartridges, and it remains unclear what spatial resolution is maintained within the visual system. We currently use two main approaches to address this question. First, we use a variety of histological techniques, including fluorescent fills (Fig.3; movie), whole tissue stains, and immunohistochemistry to investigate the shape and size of individual neurons and periodic pattern within the neuropils. The second approach uses a behavioral paradigm that tests for the spatial resolution of the motion computation system. (Fig. 4; movie).
The peculiar organization of the strepsipteran eye has led to conflicting views on its origin. We are using a comparative approach and focus on the larval and pupal development to study the eye's origin. So far all of our data suggests that the strepsipteran eye is most likely derived from compound eyes, and the eyelets have most likely evolved from single facets. We think that the eye of a strepsipteran ancestor could have experienced a reduction of the number and an increase in the size of facets to accommodate low light levels. These enlarged facets with a higher number of photoreceptors could have evolved into eyelets thereafter.
This project is currently supported by the NSF (IBN-211770). Click here to read the project summary of this grant.
The following people are currently working on this project:
- Elke Buschbeck
- Srdjan Maksimovic
Schematic comparison of an apposition type insect eye and proposed model for the function of the eye of Xenos peckii. Color is used to depict the representation of an image and does not imply functional chromatic differences. (A) In the apposition eye (a common eye type in insects) each optic unit represents only one sample point. Neighboring points of an image (arrow) are represented next to each other, at the level of the retina as well as of the lamina. (B) In the Strepsiptera, the image is viewed through many eyelets. The model illustrates the most simple case of no overlap between the visual fields of neighboring units, however some overlap is likely. As each lens is image-reversing, and only captures a partial image, the coherence of the entire image is lost at the level of the retinae, but is restored by chiasmata between retinae and the first visual neuropil, the lamina.
Confocal image of the neural projection of photoreceptor cells from one eyelet to the lamina (La).