Jennings et al.—New fossil evaniid wasp from Eocene Baltic amber


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Figure 5. Hymenoptera with modified eyes. (1) Inostemma oculare Austin (Platygastridae) paratype ♀, from India, dorso-lateral view showing small horizontal ledge in the upper quadrant of the eye. Scale line = 100 µm. (2) Isomerala azteca Girault, 1920 (Eucharitidae) ♀, from central America, anterior head showing cone-like structure of the eye. Scale line = 200 µm.


sized lenses in the miniature dipteran predatorCoenosia attenuata Stein, 1903 support resolution well below 3°, so it is conceivable that these large facets are associated with a small region of high acuity (Gonzalez-Bellido et al., 2011). That being the case, it is surprising that the largest facets are in this case found at a position that is almost the inverse of what one would expect, correspond- ing to a small radius of curvature (on the cone).Also perplexing is that the smaller facets in the transition area from the cone to the margin of the eye have a large radius of curvature. Normally in an acute zone the larger radius of curvaturewould be associatedwith larger facets. However, some caution is required in interpreting this arrangement because only the radius of curvature can be used to infer the direction that individual ommatidia view. This cannot take into account that, in many insect species, the underlying photoreceptors are displaced. Indeed, the bizarre position of a possible ‘acute zone’ is supported by a large pseudopupil visible from the frontal view of both eyes in Figure 5.2. In the cone- shaped eye structure of these species, it seems unlikely that this is not a region where the inter-ommatidial angle would be very large, yet the point-source resolution of individual facets of 23µm is potentially quite high, thus being a somewhat paradoxical design representing a likely extreme formof what has been considered to be optical under-sampling. However, extreme under-sampling is seen in other taxa, including some so-called ‘bright-zone’ eyes of Diptera (van Hateren et al., 1989) and in the peripheral eyes of jumping spiders, where it appears to be an interesting adaptation to detect small contrasting features (Land, 1985). Whether this is the case for Isomerala is unknown given little if anything is known about its biology.


Eye modifications in other insects.—Among other extant insects, the flower-mimicking hymenopodid mantids have strikingly modified eyes (Wipfler et al., 2012; O’Hanlon and Norma-Rashid, 2013). In these species, however, the vertical


tubercular extension of the dorsal eye is a chitinous structure that lacks functional ommatidia, rather than part of the eye per se. This eye modification is therefore not an adaptation that changes or enhances visual acuity, but rather is more likely associated with behavioral recognition among conspecifics or associated with procrypsis. A similar functional division as in P. oculiseparata n. sp.


between the upper (dorsal) and lower (ventral) eye is seen in a number of extant bibionid flies (Zeil, 1983), usually with some discontinuity in the transition zone between these eye regions. However, a distinct and abrupt division between the frontal and lateral eye fields, as in P. oculiseparata n. sp., is not present in these flies. A more abrupt division between the frontal and lateral eye


fields (although still not ridge-like) is exhibited by some extant hypercephalic drosophilid flies, such as Zygothrica dispar Wiedemann, 1830, Z. latipanops Grimaldi, 1987, and


Z. exuberans Wheeler, 1968 (see Grimaldi and Fenster, 1989, fig. 4), resulting in eye shapes that strongly resemble the condition found in P. oculiseparata n. sp. Interestingly, the eye modifications are only present in male Zygothrica Wiedemann, 1830. Unfortunately, the female of P. oculiseparata n. sp. is so far unknown, so we do not know whether the eye is sexually dimorphic in this fossil species. However, there is a very important difference between the eye modification in male Zygothrica and P. oculiseparata n. sp., which could be indicative of different biological functions. In these Zygothrica species the eyes are pushed antero-laterad due to the expansion of the dorsal portion of the fronto-orbital plate and the occipital sclerite (Grimaldi and Fenster, 1989), while the eyes are in the normal position in P. oculiseparata n. sp. Thus, while the eyes of these drosophilids are very similar to P. oculiseparata n. sp., and may provide a similar optical solution, they may also result from completely different selective pressures since there is no


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