Silba israel is a member of the S. admiralis McAlpine group of species characterized
amongst Silba species by having pale tarsomeres and, in the male terminalia, by having
tall, narrow cerci and a slightly protruding surstylus bearing a number of large teeth.
The group includes S. admiralis from India, S. apodesma McAlpine, and S. namibia
MacGowan from southern Africa, and S. pappi Soós from Afghanistan. There are also
a large number of undescribed species from the Congo area in Central Africa. The main
distinguishing feature between species in this group is the shape and ornamentation of
the phallus. S. israel differs from these related species in that the phallus has two flattened,
ribbon-like, lateral processes.
Head. Eye bare. Frons about half as wide as eye, parallel-sided, subshining, and covered
in dense microsculpture, on ventral half with few scattered pock-mark depressions from
which interfrontal setulae arise, dorsal half with large shallow depression just ventral
to ocellar triangle, covered in sparse very short interfrontal setulae approximately 0.15
times as long as orbital seta. Orbital plate narrow, shining black, and bare. Lunule with
6–7 setulae, dark brown ground color with silvering. Face slightly silvered. Parafacial
shining black on dorsal half and silvered ventrally beside genal setulae. Anterior genal
setulae in single row of 6–7 along mouth edge, slightly separated from other setulae on
gena, anterior group not longer or thicker than other setulae on gena. Scape and pedicel
black, 1st flagellomere mainly black, on medial surface orange on basal quarter, this coloration
sometimes extending ventrally half way along the flagellomere; 1st flagellomere
long, approximately three times as long as deep, extending ventrally beyond mouth
margin. Arista yellow basally, plumosity at its greatest extent just less than depth of 1st
Thorax. Disc subshining blue black, with short appressed setulae. Notopleural depression
bare. One propleural and one rather weak stigmatical setae. Anepisternum with
three strong setae anteriorly and three posteriorly. Katepisternum with usual two setae,
posterior seta longer and thicker than anterior seta, sclerite otherwise bare apart from
scattering of short setulae on anterior portion. Scutellar disc heavily gray microtrichose,
margin with three short setulae between lateral and apical setae, none between apical
setae. Calypteres white with white margin and fringe.
Legs. Black, foretibia with yellow streak ventrally on apical third, on all legs first and
second tarsomeres orange, fore and hind first tarsomere with fringe of golden setulae
Wing. Clear with yellow veins. Wing length 3.5–4.0 mm.
Abdomen. Male terminalia (Figs. 31–34). In lateral view epandrium 2.25 times as high
as wide, broadly excavated anterodorsally, with few well-spaced setulae along posteroventral
angle, otherwise bare. Cerci long and narrow, 0.75 times as high as epandrium,
with short scattered setulae, these densest at apex and on ventral surface. Surstylus projecting
beyond shell of epandrium posteroventrally, bearing nine relatively large teeth Phallus in lateral view approximately U–shaped, basally relatively narrow with slight
bump on basal part of U, apically with narrow, upturned central process, flanked on
each side by thin process almost as long as central process, in ventral view these lateral
processes flattened and ribbon–like, apices squared and slightly flared.
Female terminalia (Fig. 38). Aculeus dorsoventrally flattened, slightly broader than
hindtibia at its apex. Cerci with basal third embedded in shaft of aculeus, slightly acute
at tip, dorsobasally with pair of setulae less than half as long as cerci, ventrally with pair
of setulae situated on apical third of, and just longer than, cerci, and approximately 5
pairs of very short setulae on lateral margins; all these setulae whitish. Rather similar in
appearance to the aculeus and cerci of S. virescens (Fig. 39).