Ultrastructure of Scutellar Sensilla in Aphytis melinus (Hymenoptera: Aphelinidae) and Morphological Variation across Chalcidoidea

2010·11.1 MB·

by  Christina A Romero

HYM. RES.
J.
Vol. 19(1), 2010,pp. 159-178
Ultrastructure of Scutellar Sensilla in Aphytis melinus (Hymenoptera:
Aphelinidae) and Morphological Variation across Chalcidoidea
Christina A. Romero and John M. Heraty*
Department ofEntomology, University of California, Riverside, CA 92521
—
Abstract. Paired, disc-like campaniform sensilla occur on the scutellum of many minute
parasitic wasps in the superfamily Chalcidoidea (Hymenoptera). The ultrastructure ofthe sensilla
is examined in Aphytis melinus DeBach (Aphelinidae). Each sensillum consists of a bilayered
cuticular cap directly covering a tubularbody with microtubules extending at a right angle to the
cuticle. A large electron-dense mass attached to the tubular body extends laterally beneath the
cuticle.Otherstructuresoccupyingthespacebetweenthescutellumandlongitudinalflightmuscles
include the paired mesoscutello-metanotal muscles and a previously undescribed layer of oblong
structures lining the cuticle throughout the thorax. Among 23 additional species examined, the
sensillarangeindiameterfrom l.Sljimto5.79|Lim,withnoapparentrelationshipbetweendiameter
ofthe sensillaand size ofthe scutellum. Thefunctionofthe sensillais unknown,butthe consistent
presenceofthesensillainsmallchalcidoidsandthefrequentabsenceinthelargestspeciessuggests
apossibleassociationwithspecializedflightpeculiartosmallinsectsobligedtoutilizetheclap-and-
fling flight mechanism.
—
Key words. campaniform sensilla, morphology, sensory structures
Chalcidoidea is a diverse superfamily of tionally, with the vast majority of chalci-
parasitic Hymenoptera whose species doids measuring less than 4 mm, there is
range in length from the smallest known often a paucity of reliable phylogenetically
insect (0.11 mm) to relatively large wasps informative morphological structures.
(45 mm), with most specimens averaging Sensillar structures have proven to be a
mm
2-4 in length (Heraty and Gates 2003). rich source of morphological characters,
Over 22,000 species of Chalcidoidea are and there have been numerous investiga-
described, making it second to Ichneumo- tions into the structure and function of
noidea in diversity, but with an estimated sensilla found within Chalcidoidea (Baaren
100,000 to 400,000 undescribed species, it et al. 1996; Barlin and Vinson 1981; Olson
may well prove to be the largest super- and Andow 1993; Schmidt and Smith 1985,
family ofHymenoptera (Gibson et al. 1999; 1987). The antenna hasbeen the focal point
Gordh 1975a; Heraty and Gates 2003; ofthe majority ofsensillarinvestigations in
Noyes 2000, 2003). Despite over 200 years chalcidoids due to the high concentration
of taxonomic work, phylogenetic relation- and diversity of antennal sensilla (Barlin
ships at the family and subfamily levels and Vinson 1981; Basibuyuk et al. 2000;
remain unclear (LaSalle et al. 1997). Diffi- Olson and Andow 1993; Walther 1983).
culties in understanding chalcidoid phylo- These studies have focused largely on
genetics are due in part to the vast classifying types of antennal sensilla based
numbers of undescribed species and the on ultrastructural morphology (Amornsak
poor preservation of many curated speci- et al. 1998; Baaren et al. 1996; Barlin and
mens (Heraty 2004; LaSalle 1993). Addi- Vinson 1981; Basibuyuk and Quicke 1999;
Consoh et al. 1999; Isidoro et al. 1996;
"Correspondingauthor: [email protected] Olson and Andow 1993). Other investiga-
160 Journalof Hymenoptera Research
Fig. 1. Location and appearance of scutellar sensilla in Aphytis melinus. a: whole specimen, b: scutellum, c:
rightscutellar sensillum. Whiteboxes in (a) and (b) indicate area magnified in following figure.
tions have examined sensory structures of extra seta,by a pair ofsetae, orwhere there
the ovipositor (Consoli et al. 1999; Le Ralec appears to be one, three, or four sensilla in
et al. 1996; Le Ralec and Rabasse 1988; place of the normal pair of sensilla. Both
Veen and Wijk 1985), male genitalia Schauff et al. (1996) and Hayat (1998)
(Chiappini and Mazzoni 2000) and wings incorporated the placement of the sensilla
(Schmidt and Smith 1985). The paired in their keys of Encarsia Forster (Aphelini-
sensilla of the scutellum (Fig. la-c) have dae). Heraty and Polaszek (2000) used the
been identified as phylogenetically impor- close placement of the sensilla on the
tant (Hayat 1998; Heraty and Polaszek scutellum as a defining characteristic of
2000; Kim 2003; Schauff et al. 1996), but the Encarsia strenua group. Placementofthe
there has been no investigation into their sensilla on the scutellum was also used by
ultrastructure, possible function or distri- Schauff (1984) as a character in his phylo-
bution across Chalcidoidea. geny of Mymaridae. Other allusions to the
The scutellar sensilla are a feature sensilla in the literature are limited to
frequently overlooked in taxonomy and inclusion in illustrations and an occasional
have received only sparse attention in the mention in species descriptions.
literature. Domenichini (1969) was one of Herein we demonstrate that these scu-
the first morphologists working with Chal- tellar structures are campaniform sensilla,
cidoidea to point out the scutellar sensilla, which are circular to oval in shape and
noting their occurrence in several different innervated by just one sense cell, or
families and recommending that their neuron, that partially penetrates the thin-
function and taxonomic value be studied. domed cuticle (Hicks 1857; Berlese 1909;
Rosen and DeBach (1979) also noted the Snodgrass 1935; Mclver 1985). Campani-
sensilla in their treatise onAphytis Howard form sensilla have a mechanoreceptive
(Aphelinidae), mentioning in each of their function targeted at sensing tension or
species descriptions the location of the torsion in the associated cuticle (Pringle
scutellar sensilla relative to the anterior 1938a; Mclver 1985; Zill and Moran 1981).
and posterior scutellar setae. They ob- In chalcidoids, campaniform sensilla
served that, in slide preparations, the have been identified on the antenna
sensilla can be mistaken for empty setal (Amomsak et al. 1998; Olson and Andow
sockets due to the thinness of the cuticle 1993), ovipositor (Consoli et al. 1999; Le
over the sensilla. They also noted rare Ralec et al. 1996; Le Ralec and Wajnberg
mutations involving the sensilla in which 1990), male genitalia (Chiappini and Maz-
one or both sensilla are replaced by an zoni 2000), wing (Schmidt and Smith 1985;
Volume 19, Nl-mber 1, 2010 161
Weis-Fogh 1973), pretarsus (Gladun and Table 1. Abbreviations used in figures.
Gumovsky 2006) and legs (Schmidt and
Smith 1987), but the internal ultrastructure 112 longitudinal flightn:\uscles
114 mesoscutello-metanotal muscle
of these sensiUa in the superfamily has 2ph secondphragma
been examdned only in male genitalia of ass anterior scutellum setae
Mymaridae (Chiappini and Mazzoni 2000). cs campaniform sensilla
In the current study, Aphytis melimis cut cuticle
DeBach (Hymenoptera: Aphelinidae: edm electron dense mass
edr electron dense ring
Aphehninae) was chosen to examine the
ela electron lucentarea
ultrastructure of the scutellar sensilla. epd epidermal cells
Aphytis melinus range in size from fb fatbody
0.78 mm to 1.21 mm (Rosen and DeBach fl flange
LI outer layer
1979) and have been well studied because
L2 inner layer
of their success as biological agents con- m mitochondria
trolling the California Red Scale, Aonidiella ml midlinebetween left and rightlongitudinal
aurantii (Maskell) (Lenteren 1994). Prior to flightmuscles
this study, paired scutellar sensilla were ms mesoscutum
pss posteriorscutellum setae
recorded only in the smallest Chalcidoidea
scl scutellum
such as Aphelinidae (Hayat 1984, 1997;
ssr scutoscutellar ridge
Huang 1994; Heraty and Polaszek 2000; sss scutoscutellar suture
Babcock et al. 2001; Kim 2003; Noyes and tb tubularbody
Valentine 1989; Schauff et al. 1996), En- tsa transscutal articulation
cyrtidae (Hayat2003; Noyes 1988; Noyes et
al. 1997; Prinsloo 1997), Mymaridae
(Schauff 1984), Signiphoridae (Noyes and siUa on the scutellum have been termed
Valentine 1989) and Trichogrammatidae campaniform sensilla (cs), and terminology
(Doutt and Viggiani 1968; Noyes and specific to structures of the campaniform
Valentine 1989), with most attention being sensilla foUows Mclver (1985). Abbrevia-
given to scutellar sensilla in Aphelinidae tions are Hst—ed in Table 1.
(Heraty and Polaszek 2000; Kim 2003; Specimens. Aphytis melinus for scanning
Rosen and DeBach 1979). electron microscopy (SEM) and transmis-
Nopreviousworkhas soughtto examine sion electron microscopy (TEM) were
the ultrastucture of the scutellar sensilla obtained from a colony reared on Aspidio-
found in Chalcidoidea. This study seeks to tus nerii Bouche (Diaspididae) at the Uni-
survey variation in external appearance of versity of California, Riverside. An addi-
the scuteUar sensilla found in Chalcidoi- tional 30 specimens representing23 species
dea, examine the ultrastucture of the from ten families of Chalcidoidea, and one
sensilla in A. melinus, and accurately specimen of Mymarommatoidea were im-
determine the category of sensilla to which agedwith SEM. AlistofChalcidoidea used
they belong. for SEM imaging is given in Table 2; all
material is representedbyvouchers depos-
MATERIALS AND METHODS
ited at the University of California, River-
Terminology.—Terms and abbreviations side Entomology Research Museum
foUow Gibson (1997) and Kim (2003) for (UCRC). The external morphology of the
the structures of the mesonotum, Krog- sensilla in Chalcidoidea and outgroups
mann and Vilhelmsen (2006) and Vilhelm- were more broadly surveyed, but this wiU
sen (2000) for muscles and internal mor- be treated separately (Romero and Heraty,
phology, and Harris (1979) for cuticular in prep.). ScuteUar sensillae have not been
sculpturing. The paired campaniform sen- documented outside of Chalcidoidea and
162 Journalof Hymenoptera Research
Table 2. Sensillum diameters from SEM images, n indicates number ofsensilla examined for that species.
Shape of Maximum diameter Average area of
Taxon sensillum mean ± SD (range) scutellum
Aphelinidae
mm
Aphytis melinus DeBach 7 circular 4.94 |im 0.40 (3.68-4.94 ^m) 14.84
mm
Marietta sp. 4 circular 2.25 ^im 0.27 (1.93-2.60 |im) 11.18
Ablerus americanus Girault 2 circular 2.86 ^m 0.37 (2.60-3.12 ^im) 6.48 mm
Cales noacki Howard 3 circular 2.99 ^im 0.58 (2.32-3.39 ^m) 6.29 mm
Eretomocerus sp. 2 circular 4.11 ^m 0.17 (3.99^.23 ^m) 9.58 mm
mm
Eriaphytis sp. 1 circular 5.10 ^im 26.83
Encyrtidae
Comperiella bifasciata Howard 4 circular 4.10 )im 0.38 (4.14-4.83 |im) 23.71 mm
mm
Microterys nietneri (Motschulsky) 2 circular 5.35 )im 0.62 (4.92-5.79 ^im) 42.66
Eucharitidae
Orasema minutissima (Howard) 2 circular 1.87^im 0.08 (1.81-1.93 um) 27.06nm\
GoUiimieUa antennata (Gahan) 2 circular 3.47 |im 0.45 (2.24-3.79 ^m) 45.9 mm
Eulophidae
mm
Pnigalio sp. 3 subcircular 3.39 |im 0.82 (2.82-4.44 |im) 28.36
mm
Pnigalioagraules (Walker) 2 subcircular 3.93 |im 0.17 (3.80-4.05 [im) 34.43
Mymaridae
mm
Gonatocerus ashmeadiGirault 1 circular 4.87 |im 48.76
Pteromalidae
mm
Philotrypesis sp. 2 subcircular 3.70 |im 0.11 (3.78-3.62 [im) 52.76
mm
Asaphes sp. 1 subcircular 3.66 |im 30.07
NA
Nasonia vitripennis (Walker) 1 subcircular 4.34 ^im
Signiphoridae
Signiphora sp. 2 circular 2.99 ^im ± 0.20 (2.85-3.13 |im) 26.54 mm
Tanaostigmatidae
mm
Tanaostigma sp. 1 circular 3.82 |im 56.54
Tetracampidae
mm
Epiclerus sp. 1 circular 4.86 |im 17.80
Torymidae
mm
Megastigmus transvaalensis (Hussey) 1 circular 5.39 ^im 101.49
Trichogra mmatidae
Aphelinoidea sp. 2 circular 5.07nm ± 0.14 (4.97-5.17^m) 7.99 mm
Haeckeliania sp. 2 circular 5.05 nm ± 0.12 (4.96-5.13 ^m) 10.14 mm
Hayatia sp. 2 circular 3.82 ^im ± 0.30 (3.60-4.03 ^m) 5.34 mm
Total 50 3.89 ^m ± 1.01 (1.81-5.79 [im) 27.91 mm
the majority of outgroup Hymenoptera sion Proctotrupomorpha, which includes
examined had no trace of sensilla. How- Chalcidoidea, and Ceraphronidae, repre-
ever, sensilla were found in species from senting the more distantly related subdivi-
four outgroup families, Ceraphronidae sion Evaniomorpha.
—
{Ceraphron sp.), Diapriidae {Trichopria sp.), SEM. Specimens selected for SEMwere
Mymarommatidae (Mymaromma anomalum collected in 70% ethanol then dried in
(Blood & Kryger)) and Scelionidae (Tele- hexamethyldisilazane (HMDS) (Heraty
nomus sp.). These families represent three and Hawks 1998). Some specimens were
different superfamilies from the subdivi- gradually rehydrated through a series of
Volume 19, Number 1, 2010 163
increasingly dilute ethanol baths, rinsed in carbon. Sections were then post stained
two baths of deionized water, then di- using the SynapTec GridStick® system as
gested in 10% KOH for5-30min according follows. The uranyl acetate stain was
to the size ofthe specimen in order to clean diluted in methanol and the lead citrate
the specimen of debris. Specimens were stainmixedusing0.3 gramslead citrate,0.3
again rinsed in deionized water and grams lead nitrate, 0.3 grams lead acetate
dehydrated through a series of increas- and 0.6 grams sodium citrate dissolved in
ingly concentrated ethanol baths, then 24.6 ml pre-boiled double distilled deio-
chemically dried in HMDS. Once dry, nized water using a sonicator; after sonica-
specimens were either dissected or placed tion 5.4 ml of IN NaOH solution was
whole onto SEM mounting stubs. Speci- added tothelead stain. Grids were initially
mens were Au/Pd coated using a Cres- stained for 5 minutes in uranyl acetate
sington 108Auto®sputtercoater setfor 60- followed by two rinses in 100% methanol,
90 seconds, then examined and digitally one rinse each in 75%, 50% and 25%
imaged under a XL30 PEG scanning methanol, and four rinses in pre-boiled,
electron microscope at 10 or 15 kV. double-distilled deionizedwater. The grids
—
Measurements. Scutellar and sensillar were then immediately stained for 10 min-
measurements were taken in ImageJ utes in the lead stain followed by a
1.38X usingthe digital SEMimages. Width 30 second rinse in 0.02 N NaOH and
measurements ofthe scutellum were made 30 minutes of rinsing with water changed
across the broadest point of the scutellum, every 5 minutes. Sections were examined
excludingthe axillula, and lengthmeasure- with a Philips Tecnai 12 transmission
ments along the longest medial part of the electron microscope and digitally imaged
scutellum including the frenum. Area using a model 780 Gatan DualVision 300
measurements were made using the free- camera. —
hand tool in ImageJ. Measurements of the Slide Mounts. Aphytis melinus were col-
differentiated area of the sensillum were lected in 70% ethanol and gradually hy-
taken along the longest axis and excluding drated through a series of increasingly
the encirclingring, ifpresent. To determine dilute ethanol baths, rinsed in two baths
if there is a correlation between the size of of deionized water and then digested in
the scutellum and the diameter of the 10% KOH for 10 minutes. Following
sensilla, the length, width and area of the digestion, specimens were rinsed in deio-
scutella of 50 specimens were measured nized water and dehydrated through a
(Table 2). A regression line was calculated series of increasingly concentrated ethanol
for each of the three measurements of the baths to 100% ethanol. They were then
scutellum that were graphed, and the placed in a well plate with three drops of
coefficient of determination (R-squared) clove oil and the ethanol allowed to
value calculated. evaporate completely. The antennae, head,
—
TEM. Live A. melinus were decapitated wings and body were separated from each
while immersed in Karnovsky's fixative specimen and arranged on the slide in 25%
(Karnovsky 1965). After approximately Canada Balsam and 75% clove oil (Noyes
two hours they were placed in sodium 2003). As the clove oil evaporated, the
cacodylate buffer, dehydrated in ethanol Canada Balsam was gradually built up
and embedded in Spurr resin (Spurr 1969). until the structures were covered and four
mm
Sections approximately 60-70 ^im thick 5 coverslips applied.
were cut using a diamond knife on a
RESULTS
Leica Ultracut microtome. Sections were
mounted on Electron Microscopy Sciences In most Apocrita, the mesonotum is
nickel slot grids coated with formvar/ divided by the transscutal articulation
164 Journalof Hymenoptera Research
^
/^^^
Fig. 2. Structure of scutellum and scutellar sensilla inAphytis melinus. a: scutellum, b: scutellar sensillum, c:
underside of scutellum with tissue removed and both sensilla visible, d: underside of sensillum with tissue
removed. Blackarrows = 4.84fim, indicate equal distance inboth (b) and (d).
(Fig. 2a: tsa) into an anterior mesoscutum pidae and Torymidae) have a transverse
and a posterior scutellar-axillar complex sulcus or change in sculpture differentiat-
(Gibson 1997). The medially located scu- ing a posterior region of the scutellum
tellum is separated from the anterolateral termed the frenum. In many taxa, lateral
axilla by the scutoscutellar suture (Fig. 2a: axillular grooves separate the axillula from
sss). With the exception of Signiphoridae, the main portion of the scutellum, but this
in which the scutellum is reduced to a is often more apparent in lateral view. The
transverseband, all Chalcidoidea possess a scutellum of many smaller Chalcidoidea
scutellum that is a prominent plate of often has two pairs of prominent setae: the
variable size and shape. The scutellum anterior scutellar setae (Fig. 2a: ass) and
can be roughly circular, oval, shield or the posterior scutellar setae (Fig. 2a: pss).
teardrop shaped and can also vary in When present, these setae are used as
topography. For example, some Encyrtidae reference points for the campaniform sen-
have a rounded scutellum with sharply silla on the scutellum.
rising sides that form a dorsal hump, while Struct—ure of scutellum and sensilla in
some Mymaridae have a flat planar scu- Aphytis. A. melinus has a roughly oval
tellum. Many chalcidoids (i.e. Euchariti- scutellum with a pair of circular sensilla
dae, Mymaridae, Pteromalidae, Tetracam- located medially to the four primary
Volume 19, Number 1, 2010 165
scutellar setae (Fig. 2a). Each campaniform dried haemolymph which is apparent in
sensillum appears externally as a smooth the layer of "tissue'" surrounding muscle
dome inthe cuticle surroundedby a raised 114 and the sensillar stem (cs) in Fig. 3c.
ring that interrupts the imbricate sculptur- These cells line the entire internal surface
ingofthescutellum (Fig. 2b). Internally,the of the cuticle, including ventral surfaces
cuticleformsaraisedringaroundanareaof (Fig. 4i: epd) and internal apodemes
reticulate cuticle with an elliptical central (Fig. 4h: epd), but are absent where the
depression oriented diagonally to the long- scutellar sensilla attach to the cuticle
itudinal axis of the body (Fig. 2c-d). This (Fig- 4g).
—
ellipse-shaped thinning of the cuticle prob- Mesoscutello-metanotal muscles. InA. me-
ably creates a weakness along the long axis linus, a pair of muscles traverse the length
oftheellipseandenhancesmovementalong of the scutellum between the longitudinal
the short axis conferring directional sensi- flightmuscles and the dorsal surface ofthe
tivity similar to that obtained through an scutellum (Fig. 3a-b and e), which are
ellipticallyshapedcuticularcap (Moranand S5monymous with Kelsey's (1957) muscle
Rowley 1975). Across all of the specimens 114 and Vilhelmsen's (2000) mesoscutello-
surveyed,the ellipticaldepression,whichis metanotal muscle. The muscles attach to
alsovisibleinslide mounts,was found only the anterior portion of the scutellum just
inAphytis and Aphelinus (Aphelininae). posterior to the scutoscutellar ridge
Internally, the scutellum is bordered by (Fig. 3a-c: 114 and ssr). From this point of
several ridges forming a differentiated origin they narrow and are slightly angled
region directly above the longitudinal medially to a posterior insertion to the
flight muscles. Along with the mesoscu- anterior edge of the metanotum above the
tello-metanotal muscles (Fig. 3a-e: 114) margin of the second phragma (2ph) to the
and randomly distributed fat body anterior edge of the metanotum (Fig. 3a-b:
(Figs 3e: fb), this space also contains 114). In cross section, the longitudinal
several unidentified structures. In certain flight muscles have clearly defined axon
dissections examined with SEM (Figs 3c, bundles interspersed with mitochondria
4a-b), there appears to be membranous (Fig. 3d-e: 112), whereas the mesoscu-
divisions that run through this area defin- tello-metanotal muscle has mitochondria
ing irregular sections as large as 20 fxm in restricted to the periphery. Consequently
diameter, however these divisions were axon bundles are not as easily distin-
not apparent in TEM preparations. Just guished (Fig. 3d-e). These mucles may
below the cuticle, and between these affect longitudinal tension of the scutellar
divisions, there is a single, or sometimes disc and possibly deformation of shape in
double, layer of elongate epidermal cells small soft-bodied chalcidoids.
(Fig. 4a-i: epd).While tightlypacked,these Ultrastructure of the sensillar cuticular
—
cells appear independent of each other in cap. In A. melinus, there are several dis-
SEM preparations (Fig. 4a-c) and in TEM tinct features ofthe cuticular portion ofthe
preparations appear hollow due to a lack scutellar sensilla evident through electron
of penetration by the resin. Similar im- microscopy. In cross sections there is a thin
penetrable epidermal cells also appear in outer layer of solid cuticle. This layer
sections of the male antennae prepared by (Fig. 5a-e: LI) sits external to a thicker
Romani et al. (1999) in their TEM investi- layer of mesh-like cuticle (Fig. 5a-f: L2).
gationofthe male antennae ofA. melinus. It These two layers of the cap are consistent
may be that in the adult wasp the with the cuticular structure found in
epidermal cells have died leaving a thick campaniform sensilla observed in other
waxy cell membrane that is impermeable studies where 2 or 3 layer-caps are re-
to resin. These are not likely artifacts of ported (Mclver 1985) and it is nearly
166 Journalof Hymenoptera Research
i
Fig. 3. Mesoscutello-metanotal muscles in Aphytis melinus. a: underside of scutellum with most tissue
removed leavingmesoscutello-metanotal muscles (114),b: sameview in (a)with slightly differentresultsfrom
thechemicaldryingprocess,c:dorsaltissuefoundjustbeneathscutellum,d:crosssectionthroughmesoscutello-
metanotal muscle, e: cross section through dorsal portion ofscutellum.
identical to the structure observed by Moran and Rowley (1975), who called the
Bromley et al. (1980) in aphid antennae. structure a cuticular collar. Moran and
The bilayered cap is encircled by a flange Rowley also suggested that it provides
that protrudes internally. This flange was structural support and rigidity for the cap
observed by Mclver and Siemicki (1975) in of the sensilla and enables the cap to move
the mosquito, and in the cockroach by as a unit in response to cuticular deforma-
m
Volume 19, Number 1, 2010 167
Fig. 4. Epidermal cells in Aphytis melinus. a: underside of scutellum with most tissue removed leaving
epidermal cells,b: underside ofscutellumwith campaniform sensillumtissue and epidermal cells attached, c:
underside of sensilla with epidermal cells attached, d-e: cross section through scutellum, f-g: cross section
throughscutellumwithacampaniformsensillum,h: cross sectionthroughnotalridge,i: crosssectionthrough
ventralportionofmesosoma. Whiteboxes indicate area magnified infollowing figure.
tion. Just dorsal to the flange, layer 1 is nation seems appropriate since it is at this
attached to the cuticle by a ring of dark junction point, at the base of the flange
staining cuticle (Fig. 5a-c) that Mclver and where the cap and cuticle meet, that the
Siemicki (1975) called a hinge. This desig- cuticle would presumably bend.
168 Journalof Hymenoptera Research
—
Tubular body and electron dense mass. readily identified in smaller taxa such as
One of the most distinctive features of a Aphytis (Rosen and DeBach 1979). In SEM
campaniform sensilla is the tubularbody at preparations, they appear externally as
the distal end of the nerve cell (Mclver differentiated areas ofthe cuticle thatbreak
1985), which is a bundle of microtubules the cuticular pattern and typically are
set in an electron dense material that ringed by raised or depressed cuticle
functions as the site of transduction (Figs 7a-h, 8a-h).The location and shape
(Thurm 1964). In A. melinus, the tubular of the sensilla on the scutellum are highly
body is a striated cap that inserts into layer variable across Chalcidoidea, but there is
two of the cuticle, almost extending to consistencywithintaxonomic groups atthe
layer one (Fig. 5d: tb). The tubular body family, tribe, genus and species levels
consists of microtubules perpendicularly (Romero and Heraty, in prep.). The loca-
oriented to the surface ofthe cuticle and set tion of the sensilla varies from medially
in an electron-dense material. An electron- abutting in some Aphelinidae, Encyrtidae
lucent area located at the proximal end and Mymaridae, to alateral locationwithin
separates the tubular body from the elec- 5 |j,m of the edge of the scutellum in some
tron-dense mass beneath (Fig. 5c: ela and Pteromalidae and Eulophidae. Sensillar
edm). In some preparations, the tubular location also varies along the longitudinal
body appears to have a slightly indented axis from an anterior location contiguous
tip in the very center of its distal end with the scutoscuteUar suture (sss) in some
(Fig. 5e). The proximal end of the cap-like Aphelinidae and Mymaridae to a posterior
tubularbody is nested in an electron-dense locationwithina fewmicrons oftheposter-
mass (Fig. 5c: edm). This dense material iormargininsomeMymaridae.Thesensilla
surrounds the tubular body and is directly are always found anterior to the frenal line
adjacent to the modified portions of the when a frenum is present. The most
cuticular cap, completely filling the sunken common location is generally central and
areasbelow layer two and surrounding the just medial of the anterior and posterior
flange (Fig. 5b-c). It also extends beyond scutellar setae when present (Fig. 2a). The
the campaniform sensillum, particularly in shape of the sensilla range from circular
the lateral direction, to form a large matt (Fig. 7a), to longitudinally oblong (Fig. 7d),
beneath the cuticle (Fig. 6a-d). The elec- to transversely oblong (Fig. 7c), with circu-
tron-dense mass appears to consist of lar being the predominant shape. For the
microtubules or lamella similar to the subset of representative specimens mea-
tormogen cell associated with campani- sured, the diameter of the sensiUa ranges
form sensilla found on mosquito palps from 1.81)im in Orasema sp. (Eucharitidae)
(Mclver 1985), but appears to lack other to 5.79 |im in Microterys nietneri
cellular structures indicative of a tormogen (Motschulsky) (Encyrtidae) (Table 2).
cell such as a membrane bound nucleus Comparisons of sensillar diameter and
(Thurm and Kiippers 1980). No other scutellar length, width and area revealed
dendritic cells were identified in associa- that scutellar size accounts for very little
tion with the campaniform sensilla. variation in sensillar diameter (Fig. 9a-c).
Distribution of sensilla across Chalcidoi- R-squared values were low with the high-
—
dea. The paired scutellar sensilla are est value at 0.081 (Fig. 9a). Comparisons of
found in most families of Chalcidoidea scutellar length and sensillar diameter had
and in exemplars offour outgroup families an R-squared value of 0.024 (Fig. 9b), and
(Ceraphronidae, Mymarommatidae [single scutellar area and sensillar diameter had
sensillum], Scelionidae and Diapriidae). In an R-squared value of 0.057 (Fig. 9c). The
prepared slides, the sensilla appear as pale low R-squared values for the regression
spots or thin areas in the cuticle and are lines indicate that the variation in the size