Pentatomoids as Vectors of Plant Pathogens 615
other enzymatic components to break down host tissue for ingestion via the food canal. The mandibular
stylets also converge at the same location but are external to, and enclose, the maxillary stylet bundle.
This configuration of an internal maxillary stylet bundle encased by mandibular stylets forms the stylet
bundle.
In a cross-section of the stylet bundle, the apposing maxillary stylets interlink at three locations of the
maxillary bundle (top, middle, and bottom; Brozek and Herczek 2004). The top and bottom linkages pri-
marily couple the two maxillary stylets. The middle linkage functions similarly and, more importantly,
has the added purpose of delineating and separating the food and salivary canals. Thus, the location of
the middle linkage relative to the top and bottom affects food and salivary canal diameters. In hemipterans/
heteropterans, a range of configurations for these linkages occur (Cobben 1978), and the middle link-
age placement can be influenced by the type of feeding. Based on the orientation of the stylet bundle
axis, Butt (1943) indicated the possibility of a twisting aspect to the maxillary bundle along its length. A
similar twisting – or auger-type – aspect is indeed present in the stylet bundle of Nezara viridula (JFE,
unpublished data). Additionally, within the stylet bundle, the maxillary bundle itself displays a similar
apical twisting of the interlocked stylets (JFE, unpublished data).
13.1.3.3 Rostrum and Feeding Mechanics
External to the head, the stylet bundle is housed within the rostrum. The rostrum consists of the 4-segmented
labium and paired mandibular and maxillary stylets (also see Section 13.1.2, Heteropteran Feeding
Mechanisms). Several types of sensory hairs are distributed along the labium surface and at the apex of
the labium (Rani and Madhavendra 1995). These sensory hairs aid in host selection.
In resting position, the rostrum is held against the body with segment 1 lying within the bucculae on
the ventral side of the head. The stylet bundle lies within a groove in the labium but extends anteriorly
beyond segment 1. This extended portion of the stylet bundle, as well as the portion along the length of
segment 1, is almost completely enclosed by the labrum (JFE, unpublished data), which opens narrowly
by a ventral longitudinal groove. The labrum, although not part of the rostrum, still plays a vital role in
the movement of the stylets during feeding (see below).
When investigating a potential food source, the insect keeps the labrum against the head venter.
However, as the insect begins to insert its stylets into the host, it retains segment 1 within the bucculae
but articulates segment 2 away from the insect venter, thereby forming variable angles between segments
1 and 2 when feeding (Esquivel 2011); simultaneously, the labrum (and enclosed portion of stylet bundle)
also articulates away from the insect venter and the stylets are extracted from segments 1 and 2 to align
the stylet bundle with segment 3. Segment 3 articulates at the juncture with segment 2, and this latter
articulation maintains segments 3 and 4 perpendicular with the food source. The latter segments also
aid in protecting and guiding the stylet bundle. None of the labial segments penetrates the food source.
The insect not only mechanically protracts and retracts the stylet bundle into and from the food source,
respectively, it also incorporates its entire body as leverage for penetration (JFE, personal observation).
13.1.3.4 Stylet Penetration Potential
Historically, staining of feeding locations has been used to estimate the depth of stylet penetration by
pentatomids and other hemipterans (e.g., Smith 1926). Esquivel (2011), however, devised a practical
mathematical model to calculate penetration estimates for pentatomids and mirids (Esquivel 2015) with-
out the use of the labor-intensive staining methods. This new model primarily relies on the interaction
between labial segments 1 and 2, as well as the mechanics of the labrum (described above), when an adult
Nezara viridula assumes a feeding position. For illustrations and mathematical details of the model, the
interested reader may consult Esquivel (2011).
For Nezara viridula, stylet penetration was significantly deeper as the angles between segment 1 and
the proximal end of segment 2 became more acute (Esquivel 2011). At all angles tested (90° — 50°),
males differed significantly from females, and all instars differed significantly from one another. Length
of rostra and individual segments increased proportionally as the insect progressed through stadia to
adulthood (Esquivel 2011). Similar measurements of other adult pentatomids indicated that longer rostra