Table Of ContentPROC. ENTOMOL. SOC. WASH.
108(1), 2006, pp. 210-225
A QUANTITATIVE EVALUATION OF INTRASPECIFIC AND
INTERSPECIFIC VARIATION IN EYE MORPHOLOGY FOR EFFERIA
COQUILLETT (DIPTERA: ASILIDAE) SPECIES
K. C. HOLSTON AND C. R. NELSON
(KCH) Swedish Museum of Natural History, Section for Entomology, P.O. Box 50007,
S-104 O05 Stockholm, Sweden (e-mail: [email protected]); (CRN) Department of In-
tegrative Biology, WIDB 401, Brigham Young University, Provo, UT 84602, U.S.A. (e-
mail: [email protected]).
Abstract.4Qualitative taxonomic characters based on differences in linear eye mea-
surements were converted to qualitative scores for Efferia group species, and statistical
analyses of intraspecific and interspecific variation were perfomed. Sexual dimorphism
noted for E. albibarbis is the first documented for an eye morphology character in Asi-
lidae, and may be analogous to sexual dimorphism described in a holoptic-dichoptic con-
text for other species of Diptera. Interspecific variation in E. albibarbis Macquart and E.
aestuans (Linnaeus) demonstrates that robust comparisons among species cannot rely on
single exemplars, and differences among specimens based on sex and geography should
be considered during subsequent eye morphology studies in Asilidae. The variables iden-
tified in this study demonstrate that a wide variation in eye morphology exists among
Efferia group species that can be examined in a quantitative context. Detailed accounts
of intraspecific variation are considered crucial for comparisons among species of Asili-
dae, and interspecific comparisons of eye morphology are likely to demonstrate biologi-
cally significant results in studies of monophyletic taxa and ecological guilds.
Key Words: intraspecific variation, eye morphology, robber fly, Efferia albibarbis, Ef-
feria aestuans
Compound eyes of robber flies (Diptera: noted concerning hovering ability, habitat
Asiloidea: Asilidae) are sensory organs of preferences, and prey handling. The com-
great importance for these active visual pound eyes of robber flies are large and
predators, and studies of eye morphology prominent with respect to dorsal and ventral
are relevant to further taxonomic, ecologi- margins of the head in anterior view, such
cal, and phylogenetic assessments of Asi- that the vertex appears <8excavated=9 (Wood
lidae. Robber flies hunt primarily from ex- 1981, Lehr 1988, Majer 1997). Eyes in both
posed perches on vegetation or the ground, sexes are well-separated at the vertex
where they often move their heads and re- (Wood 1981), which has been considered
orient their bodies in response to move- an evolutionarily derived characteristic for
ments of potential prey. The subsequent at- Asilidae (Yeates 1994), although holoptic
tack flight has been described as <8target- males (i.e., the margins of the compound
directed== or 8<8ballistic interception99 eyes meet dorsally at vertex) and dichoptic
(Buschbeck and Strausfeld 1996, 1997), females (i.e., compound eyes separated) fre-
and differences among species have been quently characterize species of Diptera
VOLUME 108, NUMBER 1 2a
(McAlpine 1981). The predatory behavior would provide an informative reference for
of robber flies may promote similar eye further studies.
morphology between males and females, Face, eye, and vertex widths have been
but sexual dimorphism has been noted for reported in descriptions of robber fly spe-
other body parts directly involved with pre- cies and can be expressed as ratios that de-
dation, such as wings and legs (Hull 1962, scribe the relative proportions of these mea-
Lavigne 2002). surements. Qualitative taxonomic diagnos-
Intraspecific differences in eye morphol- tic characters used in keys, such as <8face
ogy, however, have not been documented, wide, about width of eye= or <8frons greatly
and interspecific differences are generally expanded toward vertex99 are based on the
summarized as qualitative taxonomic char- relative widths of these three features
acters to differentiate among genera. The (Wood 1981). The distance between the
degree to which the compound eyes con- compound eyes across the face (88face
verge toward the vertex is often used in tax- width99) and across the vertex (8vertex
onomic characterizations of Asilidae, with width99) and the width of the eye are mea-
the vertex width relative to face width used sured from endpoints on the eye margin;
to distinguish genera having compound these measurements describe, therefore,
eyes that are widely divergent towards the three major aspects of the anterior eye mor-
vertex (e.g., Stichopogon, Lasiopogon, phology. Ratios derived from these linear
Townsendia, Willistonina) (Wood 1981). measurements yield values corresponding
Quantitative variables for eye morphology directly to these taxonomic characters that
would facilitate precise comparisons among eliminate the effect of specimen size from
specimens and species and allow eye mor- comparisons. For example, if face and eye
phology to be incorporated in biological in- width are equal, then the 1:1 ratio can be
vestigations of robber fly species. expressed as the quotient of face width/eye
The wide geographical distribution and width, or 1.0; a 1:2 ratio for a specimen
abundance of Efferia albibarbis (Mac- with eye width twice that of the face yields
quart), and its distinctive eye morphology a value of 0.5.
makes this species an appropriate choice for In this study, interspecific and intraspe-
describing intraspecific variation in Efferia. cific differences in relative face, eye, and
Fisher and Hespenheide (1992) considered vertex widths for Nearctic Efferia species
E. albibarbis the most common robber fly were examined to evaluate the utility of eye
in North and Central America, and this spe- measurements in a quantitative context.
cies 1s often well represented in collections
of North American Asilidae. Wilcox (1966) MATERIALS AND METHODS
noted in his key to the <8Albibarbis Group= Nomenclature.4Although the Nearctic
that E. albibarbis has a <8frons at vertex as Efferia species featured in this study have
wide as at antennae= in contrast to the nar- been divided recently among seven genera,
rower vertex in other Albibarbis Group spe- these genera represent groups of Efferia
cies. Wilcox (1966) also reported eye, face, species considered artificial groups by pre-
and vertex widths for 81 Nearctic Efferia vious authors (Hine 1919, Wilcox 1966,
species, but the utility of these measure- Martin and Papavero 1970, Fisher and Hes-
ments in taxonomic or biological studies penheide 1992, Artigas and Papavero
has not been evaluated. Efferia aestuans 1997). Name combinations for species cor-
(Linnaeus) is a North American species that respond to the genus level classification of
is also well represented in collections, and Asilidae by Fisher and Hespenheide (1992)
a comparison of eye morphology among Ff- such that all species names are presented in
feria species with emphasis on intraspecific combination with Efferia except Triorla in-
variation in E. albibarbis and E. aestuans terrupta (Macquart). The genus names re-
PROCEEDINGS OF THE ENTOMOLOGICAL SOCIETY OF WASHINGTON
i) in)
euifse Ue Diagram of a robber fly head (E. albibarbis, anterior view with antennae and upper setae of head
removed), showing the three distances measured (a = width of vertex, b = width of face, c = width of left
eye).
cently proposed for Efferia species groups Natural History Survey (INHS) (Fig. 1).
or removed from synonymy with Efferia by Localities from which specimens were col-
Artigas and Papavero (1997) are not used. lected were grouped by geographic area: 1)
The term <Efferia group species= is used, Midwestern United States (MWUSA), 2)
therefore, in reference to the Nearctic Ef- Baja California Norte, Mexico (BAJA), 3)
feria species plus T. interrupta. Names and Central America (CAMER), and 4) south-
circumscriptions for species groups in Ef- western United States (SWUSA), (Table 1).
feria follow Wilcox (1966) except that 7. Measurements of the head in anterior view
interrupta is considered separately from Ef- were recorded using an ocular micrometer
feria and Staminea group species. at 50X and are reported as ocular micro-
Eye morphology measurements.4Mea- meter units (om).
surements for the left compound eye, face, Wilcox (1966) reported measurements of
and vertex widths were recorded for 103 the eye, face, and vertex for 81 Nearctic
specimens of E. albibarbis and 48 speci- Efferia group species in ocular micrometer
mens of E. aestuans housed at the Illinois units at a magnification of <about 30
VOLUME 108, NUMBER 1 218
Table 1. Localities from which specimens of E. be evaluated based on predicted patterns.
aestuans and E. albibarbis were examined, grouped by Differences in eye width determine the
the geographic region noted for comparisons. Legend:
range of variation in CDI scores and the
MWUSA = midwestern USA, SEUSA = southeastern
USA, SWUSA = southwestern USA, CAMER = Cen- placement and spread of observations in
tral America, BAJA = Mexico, Baja California Norte. correlation plots.
Pattern 1: If face, vertex, and eye width
MWUSA Indiana (Clinton), Illinois (Adams, Bond, measurements are constant or proportional
Brown, Champaign, Dubois, Eddyville, Edgar, Ef- among specimens, then face/vertex and CDI
fingham, Hancock, Iroquois, Jackson, Marion, Mc-
scores will be identical and all observations
Henry, McLean, Pope, Pulaski, Richland, Scott, St.
share a single point in the correlation plot.
Clair, Wabash), Michigan (Berriens).
SWUSA Arizona (Mohave, Pima, Santa Cruz), New Pattern 2: If only eye width varies and
Mexico (McKinney, Otero). face and vertex widths are constant or pro-
CAMER Guatamala (Esquintla, Quiche), El Salva- portional among specimens, then face/ver-
dor (Cajunte), Costa Rica (Guanacaste, Puntaren-
tex scores will be identical and the distri-
as).
bution of observations is linear and perpen-
BAJA Baja California Norte.
dicular to the x-axis.
Pattern 3: If eye width is constant and
face and vertex widths increase arithmeti-
times.99 Measurements were recorded for a cally (e.g., face and vertex increase by 2
single male specimen of most species; a fe- omu), then CDI scores will be identical and
male was measured for E. femorata (Mac- the distribution of observations is linear and
quart) and E. clementi (Wilcox and Martin). perpendicular to the y-axis. This pattern
Eye morphology variables.4We have also characterizes groups with identical dif-
used four variables to compare differences ferences in face/eye and vertex/eye scores.
in eye morphology among specimens, using Pattern 4: If vertex and eye widths are
ratios derived from linear measurements to constant or proportional and only face
control for differences in specimen size. A width varies, then the distribution can be
straightforward quantitative description of described by the following linear equation:
eye convergence toward the vertex is the y = (vertex/eye)x4(vertex/eye). The slope
face width across the antennal bases divid- of this line is m = vertex/eye, the y-inter-
ed by the vertex width (face/vertex). An ad- cept is b = 4vertex/eye, and the x-intercept
ditional description of eye convergence is is at (1,0). <Pattern 4= also characterizes
obtained by subtracting the face width di- groups of specimens having arithmetic in-
vided by eye width (face/eye) from the ver- creases in face, vertex, and eye widths. Dis-
tex width divided by eye width (vertex/eye). tributions become more curvilinear as eye
This <8Convergence-Divergence Index= measurements change in larger increments,
(CDI) is distinguished from face/vertex by and the closest fits are expected for small
its use of two variables that incorporate eye incremental increases.
width (CDI = face/eye4vertex/eye). Scores An additional characteristic of face/ver-
showing low or no variability within and tex and CDI correlation should be noted.
strong differences among groups indicate The point indicating equal face and vertex
variables that are most appropriate for tax- widths is (1, 0), where the face/vertex score
onomy. equals 1 and the CDI equals 0. As the
Proportional change in eye morpholo- widths of the face and vertex become in-
gy.4The relationship between face/vertex creasingly similar (e.g., face/vertex 4> 1),
and CDI scores allows correlation plots to differences in CDI scores for an identical
depict proportional changes in eye width range of eye widths are smaller due to clos-
among specimens examined, which allows er face/eye and vertex/eye scores near the x-
the variables describing eye convergence to intercept. The fit of observations in a face/
214 PROCEEDINGS OF THE ENTOMOLOGICAL SOCIETY OF WASHINGTON
vertex and CDI correlation plot to the linear E. albibarbis indicate moderate levels of
equations described above would be closer variation in the variables studied, with no
near the x-intercept even when overall var- overlap in face/vertex and CDI scores (Ta-
iation in eye width does not change. ble 2). The face/vertex and CDI scores are
Statistical descriptions and analyses.4 higher for E. aestuans than for E. albibar-
Box plots were constructed to summarize bis, showing that the compound eyes in E.
descriptive statistics for Efferia group spe- aestuans (face/vertex: 1.1020 to 1.2745,
cies, with inner fences for the data extend- mean = 1.1910 + 0.0408; CDI: 0.0806 to
ing | step beyond the interquartile range. 0.1872, mean = 0.1326 + 0.0241) con-
Correlation plots were developed to de- verge more strongly towards the vertex than
scribe the relationship between the face/ver- for E. albibarbis (face/vertex: 0.8841 to
tex ratio and CDI, with linear regressions 1.0526, mean = 0.9745 + 0.0434; CDI:
calculated in Microsoft® Excel 2000, ver- =(01228 to9 0:0476, mean9 = 40.02427"=
sion 9.0. Pairwise comparisons between 0.0413). The face/eye and vertex/eye scores
groups of specimens measured during this show that eye width is generally greater
study (separated by species, geographic re- than face or vertex widths in both E. aes-
gion, and sex) were performed using un- tuans (0.7037 to 0.9643, mean = 0.8345 +
paired two-tailed ¢-tests. These comparisons 0.0531; vertex/eye: 0.5977 to 0.8750, mean
test for significant differences in eye mor- = 0.7019 = 0.0576): and E. albibarbis
phology scores between EF. albibarbis and (face/eye: 0.7746 to 1.0678, mean = 0.901 1
E. aestuans and between conspecific males + 0.0642; vertex/eye: 0.8060 to 1.0702,
and females. mean = 0.9225 + 0.0620) and that E. al-
bibarbis has more similar face, vertex and
RESULTS eye widths.
Intraspecific variation.4In E. aestuans, The linear relationship between face/eye
there is a higher correlation between face and CDI scores indicates a trend toward
and vertex widths in males (y = 0.6901x + proportional change in eye measurements
8.6234, R? = 0.8902) than females (y = for E. albibarbis and E. aestuans following
0.6956x + 8.0324, R? = 0.8879), a lower the <Pattern #4= distribution, but variation
correlation in face and eye for males (y = in scores shows that increases are arithmetic
1.4152x + 11.518, R*? = 0.6644) than fe- (Fig. 2). The distribution for all E. albibar-
males (y= 01.4298x. 13.858,> R2 = bis specimens can be described by a linear
0.8420), and a lower correlation in vertex regression (y = 0.9548x 4 0.9547, R? =
and eye widths in males (y = 0.3046x + 0.9941) with a slope similar to the observed
26.632, R? = 0.5230) than females (y = mean (0.9547 + 0.0513) and median
0.4063x + 19.776, R? = 0.7354). In E. al- (0.9547) vertex/eye scores. Similarly, the
bibarbis, there is a lower correlation be- distribution for E. aestuans can be de-
tween face and vertex widths in males (y = scribed by a linear regression (y = 0.6987x
0.9942x + 1.4494, R? = 0.6968) than fe- 4 0.7, R* = 0.8225) close to the observed
males (y = 0.8615x + 7.6243, R? = mean (0.7019 + 0.0576) and median
0.9048), a higher correlation in face and eye (0.6992) vertex/eye scores. The correlation
formmaless(y°4 1037x511 9346 SRA 4 plot for face/eye and CDI shows lower fit
0.7584) than females (y = 0.7326x + to a linear regression for E. aestuans than
17.986, R? = 0.4741), and a higher corre- for E. albibarbis although higher standard
lation in vertex and eye widths in males (y deviations are observed for E. albibarbis,
=O 1x 14.957, RJ 410:5824) ethan which is predicted from the calculation of
females (y = 701541797x04; 925.768; -R204 the CDI for distributions closer to the x-
OS9Siy). intercept (see <<Materials and Methods99).
Intraspecific variation in E. aestuans and Unpaired two-tailed ftests reveal signif-
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216 PROCEEDINGS OF THE ENTOMOLOGICAL SOCIETY OF WASHINGTON
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Fig. 2. Correlation plot for face/eye and CDI scores, showing separation of distributions and <8Pattern 4=
linear trends for E. albibarbis (triangles4lower scores) and E. aestuans (diamonds4higher scores).
icant differences between E. albibarbis and face/vertex and CDI scores for males)
E. aestuans (P < 0.001) and significant in- whereas there is a complete overlap of
traspecific differences for E. albibarbis (Ta- scores for males and females of E. aestuans
ble 3). Differences between males and fe- (Fig. 3, Table 3). More females than males
males were not significant for E. aestuans have face/vertex scores greater than 1.0
(P > 0.413) but significant between males (Nynrates = 14, Ngemates = 18), more males have
and females of E. albibarbis in face/vertex face/vertex scores between 1.0 and 0.965
scores (P = 0.014) and nearly significant in (Males = 11, Memales = 3), and only males
face/vertex scores (P = 0.062). Males and have scores below 0.965 (jae; = 35 Deemates
females from MWUSA constitute the larg- = (0).
est geographic group of E. albibarbis spec- Interspecific variation.4The four eye
imens, and have a higher significant differ- morphology variables identify outliers rath-
ence in face/vertex (P = 0.004) and are sig- er than discrete groups of Efferia species,
nificantly different in face/eye (P = 0.008) and all but two Efferia group species (7.
and CDI scores (P = 0.003). Significant interrupta and E. albibarbis) have com-
differences between males and females in pound eyes that are closer at the vertex than
face/vertex scores were also detected for at the frons (i.e., face width > vertex width)
BAIA (2 4 .0:041)eand CAMER (2 4 (Fig. 4, Table 4). The face/vertex ratio and
0.043) groups and in CDI scores for the CDI indicate divergence of the eyes at the
CAMER group (P = 0.040). Summary sta- vertex for 7. interrupta, neither divergence
tistics and correlation plots of face/eye and nor convergence for E. albibarbis, and con-
CDI scores show that scores for MWUSA vergence for all other Efferia group species.
E. albibarbis males extend farther into the The lowest face/vertex scores are recorded
realm of divergent eye scores (i.e., lower for T. interrupta (0.980) and E. albibarbis
VOLUME 108, NUMBER 1 PLING
CDI
face/vertex
CDI
1,20
face/vertex
Fig. 3.
Correlation plots of face/eye and CDI scores for MWUSA E. albibarbis (upper plot, triangles) and
E. aestuans (lower plot, diamonds) comparing distributions of males (solid symbols) and females (open symbols),
with lower scores for males than females in E. albibarbis.
(1.000); and the highest scores are observed aestuans include scores calculated from
for E. leucocoma (Williston) (1.500), E. Wilcox (1966) measurements. There is,
zonata (Hine) (1.500), and FE. peralta Wil- however, broad overlap in distributions of
cox (1.600), with E. peralta as an upper scores for the eight Efferia species groups
outlier among Efferia group species (Fig. in all eye morphology variables (Table 5),
4). The lowest CDI scores were also ob- and the ranges of face/vertex and CDI
tained for 7. interrupta (-0.0189) and E. al- scores for E. albibarbis and E. aestuans en-
bibarbis (0.000), but E. leucocoma has the compass scores for 7. interrupta and 37
highest CDI score (0.308) with 7. interrup- other Efferia species (Fig. 5).
ta, E. albibarbis, and E. leucocoma as out- The correlation plot for Efferia group
liers for the CDI (Fig. 4). The face/vertex species shows a linear relationship for face/
and CDI scores for E. albibarbis and E. vertex and CDI scores, and the slope of the
218 PROCEEDINGS OF THE ENTOMOLOGICAL SOCIETY OF WASHINGTON
Table 3. Results (p-values) from unpaired two-tailed t-tests of face/eye, vertex/eye, face/vertex, and CDI
scores; with comparisons between E. aestuans and E. albibarbis, between all males and females of E. aestuans
and E. albibarbis, and between males and females of E. albibarbis grouped by geographic region. Significant
differences between groups (~ = 0.05) are indicated by asterisks (*).
Face/Eye Vertex/Eye Face/Vertex CDI
E. aestuans vy. E. albibarbis <0.001* <0.001* <0.001* <0.001*
E. aestuans v. E. albibarbis, 3 3 <(0.001* <0.001* <0.001* <0.001*
E. aestuans vy. E. albibarbis, 2 & <0.001* <0.001* <0.001* <0.001*
E. aestuans, 66 v. E. aestuans, 2 & 0.835 0.882 0.576 0.413
E. albibarbis, 33 vy. E. albibarbis, 2 & 0.062 0.741 0.014* 0.203
MWUSA, 66 v. MWUSA, @ 2 0.008* 0.695 0.004* 0.003*
BAJA, 66 v. BAJA, 22 0.017* 0.522 0.041* 0.158
CAMER, 66 v. CAMER, @ 2 0.695 0.130 0.043* 0.040*
SWUSA, 36 v. SWUSA, 22 0.687 0.351 0.072 0.075
regression line through the x-intercept with tuans and E. albibarbis, the correlation plot
best fit to: the data (Gy 4 0:6227x 4 0:625, shows greater spread of observations with
R2 = 0.6996) has a value lower than me- increasing distance from the x-intercept,
dian (0.6739) and mean vertex/eye scores and vertex/eye scores indicate a lack of pro-
(0.6651 + 0.1209) (Fig. 6). Efferia species portionality of vertex and eye widths
with the best fit to this linear regression among Efferia group species.
have vertex/eye scores that are closest to
DISCUSSION
Ol6227 (EZ. adeci: 0.6000, E. wilcoxi:
0.6207, E. latrunculata: 0.600, E. inflata: Intraspecific variation and interspecific
0.6000, E. pilosa: 0.6250, E. kansensis: distinctions.4All four variables can be
0.6000, and E. leucocoma: 0.6154) (Table used to distinguish specimens of E. albi-
4). As seen in the regressions for E. aes- barbis from E. aestuans. The significant
H =
0,95 - 4 1,05 4113 1a1 -25 <es 1,45 1=,| 5e5 a 1,61 5
= a =
lon a T <= SS ia ea = 4 = = T r 1
-0,05 oO 0,05 0,1 0,15 0,2 0,25 0,3 0,35
Fig. 4. Boxplots summarizing distributions of face/vertex (upper plot) and CDI (lower plot) scores, as listed
for all species in Table 4. Symbols: face/vertex4T. interrupta, 0.980 (first square); E. albibarbis, 1.000, (tri-
angle), E. cana, 1.070 (second square); E. aestuans, 1.143 (diamond); E. leucocoma and E. zonata, 1.500 (third
square); and EF. peralta, 1.600 (upper outlier, fourth square). Symbols: CDI4T. interrupta, 40.0189 (lower
outlier, first square); E. albibarbis, 0.000 (lower outlier, triangle); E. cana, 0.062 (second square); E. aestuans
(0.116, diamond); E. peralta, 0.267 (third square); and E. leucocoma, 0.308 (upper outlier, fourth square).
VOLUME 108, NUMBER 1 219
differences in eye morphology emphasize scores are due to larger vertex and eye
that FE. albibarbis and E. aestuans are dis- widths in males of E. albibarbis rather than
tinct species although the taxonomic status narrower face widths. Face width and eye
of these species is clearly established using widths are related by a shared endpoint on
a suite of traditionally used morphological the eye margin (Fig. 1), and a narrower face
characters (Wilcox 1966). Similarly, E. al- width that is not complemented by a larger
bibarbis and other species that are outliers eye width would result in sexual differences
in distributions of eye morphology scores in ratios of overall head width (face width
can be diagnosed without reference to the + eye width) to vertex width as in face/
variables defined in this study. These vari- vertex scores. To investigate this possibility,
ables, however, are more informative de- two tailed t-tests for (facet+eye)/vertex
scriptors of eye morphology than summa- scores were performed. A lack of signifi-
ries of linear measurements (e.g., the aver- cant difference in (face+eye)/vertex scores
age eye width for specimens examined), between males and females from MWUSA
which lose diagnostic properties that are in- (P = 0.334) or all E. albibarbis specimens
dependent of specimen size. Further docu- (P = 0.186) shows that lower scores for eye
mentation of eye measurement ratios can divergence variables in males correspond to
improve the scope of current species diag- increases in both eye and vertex widths. An
noses, but ratios should be selected based arithmetic increase in both vertex and eye
on their relevance to further taxonomic and width explains the lack of significant dif-
biological investigations. ference between males and females in ver-
Intraspecific sexual differences.4The tex/eye scores. Furthermore, a positive cor-
lower minimum, median, and mean face/ relation between face and eye widths in E.
vertex, CDI, and face/eye scores in male albibarbis demonstrates that eye width in-
specimens of E. albibarbis indicate differ- creases with increasing face widths, and
ences in eye morphology between males males have a stronger positive correlation
and females. Significant sexual differences in vertex and eye widths than females. The
were detected in face/vertex, CDI, and face/ correlations between face/vertex and CDI
eye scores, indicating that males have either scores also indicate that face, eye, and ver-
a narrower face or broader vertex and eye tex measurements increase arithmetically:
in comparison to females. The correlation the fit to a linear regression is not due to
between face and vertex widths for males proportional changes in vertex/eye scores.
of E. albibarbis indicates that face and ver- Sexual differences in eye morphology
tex widths do not increase arithmetically as have not been documented in previous stud-
face and vertex widths for E. albibarbis fe- ies of Asilidae and suggest that directional
males or for either sex of E. aestuans. Mod- selection in E. albibarbis may have shifted
erate variation in vertex/eye scores was ob- face/vertex, CDI, and face/eye scores to
served for E. albibarbis and E. aestuans, lower means in males. The distribution of
and there is no significant difference in face/vertex and CDI scores for fly species
means of vertex/eye scores between males with holoptic males would be bimodal and
and females MWUSA specimens (P = non-overlapping between males and _ fe-
0.695) or in comparisons of all E. albibar- males. Eye morphology in robber fly spe-
bis specimens (P = 0.741) (Table 3). Males cies 1s considered identical between males
and females of E. aestuans, in contrast, are and females, and the predicted distribution
not significantly different in scores for any of face/vertex and CDI scores would be un-
of the four eye morphology variables (P = imodal, with complete overlap and identical
0.413) despite a lack of proportional change means for male and female distributions. In
in eye measurements (Table 3). E. albibarbis, however, males have lower
Lower face/vertex, CDI, and face/eye median and mean scores than females, with
