The diversity, evolution, and development of setal morphologies in bumble bees (Hymenoptera: Apidae: Bombus spp.)

Abstract

Bumble bees are characterized by their thick setal pile that imparts aposematic color patterns often used for species-level identification. Like all bees, the single-celled setae of bumble bees are branched, an innovation thought important for pollen collection. To date no studies have quantified the types of setal morphologies and their distribution on these bees, information that can facilitate understanding of their adaptive ecological function. This study defines several major setal morphotypes in the common eastern bumble bee Bombus impatiens Cresson, revealing these setal types differ by location across the body. The positions of these types of setae are similar across individuals, castes, and sexes within species. We analyzed the distribution of the two most common setal types (plumose and spinulate) across the body dorsum of half of the described bumble bee species. This revealed consistently high density of plumose (long-branched) setae across bumble bees on the head and mesosoma, but considerable variation in the amount of metasomal plumosity. Variation on the metasoma shows strong phylogenetic signal at subgeneric and smaller group levels, making it a useful trait for species delimitation research, and plumosity has increased from early Bombus ancestors. The distribution of these setal types suggests these setae may serve several functions, including pollen-collecting and thermoregulatory roles, and probable mechanosensory functions. This study further examines how and when setae of the pile develop, evidence for mechanosensory function, and the timing of pigmentation as a foundation for future genetic and developmental research in these bees.

Keywords: Bee; Bombus; Development; Morphology; Pigmentation; Pollen; Pollination; Setae; Systematics.

Figure 1. The major setal types on the bumble bee body.

Each major type occurs in a separate outlined box, with a close up photo of the type and a diagram of the body shading where that type occurs along the body. Plumose and spinulate have long and short subtypes shown. Images of the body include a diagram of the ventral side on the left half (leg positions and tergite lateral extensions outlined) and the dorsal side on the right (eye, face, and vertex regions outlined in the head), and on the far right shows a lateral view of the pleuron. Shading reflects where the setae of that type occur, although density levels vary. Bar = 100 uM.

Figure 2. Images of setal morphology diversity and pigmentation.

(A) SEM image of B. bimaculatus setae located on the face, just below the antennae, showing both spinulate (B) and plumose (P) hair types. (B) Light microscope image of shaved setae from the posterior-lateral thorax of B. impatiens, showing spinulate, plumose (long and short), and bipentunculate (E) setal types. (C) Simple setae (lighter colored) on the metasomal sternites of B. impatiens. (D) Magnification of the black (top) and yellow (bottom) setae of B. impatiens when near full pigmentation. Middle: SEM image showing fine ribbing on setae (B. bimaculatus). (E) Black setae of B. impatiens imaged at the 0hr callow stage showing how color progresses from the base of the seta towards the tip and how percent darkness can be assessed in these hairs using image data. (F) Keirotrichia of the posterior (inner) side of the hind tibia with a close-up view of these setae on the right.

Figure 3. The distribution of plumose setae on the head, mesosomal, and metasomal tergites in females in representative species (n = 127/~260) across the phylogeny of bumble bees.

This shows the percentage of plumose setae relative to spinulate setae in each region. Only metasomal tergites 1–5 are shown. Most codes are based on 2–3 individuals per species and are based largely on worker phenotypes. Intraspecific variation is represented by partial codings for respective sections. The overall percent of the body with >50% of plumose setae for each species and from each segment combined across species is represented in grayscale. The bee at the upper left shows the directionality of increased plumosity for each segment, showing how when plumose setae are of low percent they occur in the posterior of the tergite and when percentages increase, plumosity extends towards the anterior, especially in the middle.

Figure 4. Temporal patterns of developing setae in the bumble bee B. impatiens.

(A–D) Stage P2, P4, P5 and P7, respectively, with setal morphology for each stage diagrammed at top and a pupal image for the respective stage at the bottom. Tr, trichogen; To, tormogen; Ep, regular epidermal cells. Dotted lines outline the hair shaft. Small figures in (C) and (D) represent zoomed in views of spinulate setae, showing blunted branches at P5 and pointed branches at P7. The small figure in (E) represents the appearance of the tips of plumose mesasomal setae around P5. All samples except (E) are prepared from metasomal tergite 2, where spinulate setae are present. Scale bar = 40 µm.

Figure 5. Cellular pattern of branched setae on T2 in B. impatiens ((A–D) TEM, (E and F) CLSM).

(A and B) Tubular bodies, (C–E) setal bases with adjacent epidermal cells (ds, dendritic sheet; tb, tubular body; mt, microtubules; tri, trichogen cell; the, thecogen cell; cut, cuticle; bl, basal lamina; tor, tormogen cell; acc1, 2, accessory cells; sc, setal cuticle).

Figure 6. Temporal pigmentation pattern of cuticular setae in bumble bee B. impatiens from 0–72 h post adult emergence “eclosion”.

(A) Pigmentation of the black setae is measured as relative darkness of the developing setae to the level of the fully melanized adult (72 h post adult emergence). (B) Pigmentation process of the yellow setae, represented by spectrophotometer absorbance of pigment solution extracted from the developing setae. Note that in the black setae, setal color already reaches 50% of adult intensity at the time of eclosion, and achieves ~90% full adult melanization by 24 h, whereas yellow setae start pigmentation around 12 h post adult emergence and complete pigmentation around 32 h post emergence.