3.1. Antennae
3.1.1. Gross morphology of the antennae
The antennae of A. tumida compose of 11 segments, including scape, pedicel and 9 flagellums (Figure 1). Antenna usually places in the antennal groove on the ventral surface during resting. Scape connects with head through antennifer, and with outer margin obtuse. Pedicel is cylindrical and constricted basally. Flagellums 1–6 gradually become shorter and wider, and flagellum 6 is almost disc-like. The last three segments of flagellums are swollen and gradually narrower, collectively known as uncompact antennal club. The lengths of the male and females are 975.6 ±15.0μm and 972.9±8.0μm, respectively (Table 1), and there are no significant sexual differences in the length of each segment (P > 0.05). Antennal club has the most abundant types and quantity of sensilla, especially on the apex of terminal segment, followed by the scape and pedicel, flagellums 1–6 are least.
3.1.2. Sensilla on antennae
5 types and 13 subtypes of sensilla were identified on antennae: 3 subtypes of sen-silla chaetica (SC), 6 subtypes of sensilla basiconica (SB), 1 type of sensilla trichodea (ST), 2 subtypes of sensilla styloconica (SS) and 1 type of Böhm bristles (BB). The division standards of antennal sensilla are presented in Table S1. The number and types of sensilla on the dorsal side of the antenna are more than those on the ventral side, and the antennae club has the most abundant types and numbers of sensilla.
Sensilla chaetica (SC) are long setae or spines, usually perpendicular to the cuticula and higher than other sensilla. They are divided into three subtypes (SC1, SC2 and SC3) ac-cording to the length and surface grooves.
Sensilla chaetica 1 (SC1) are the longest and most numerous long setae. They have sharp tips, longitudinal grooved walls and scattered into wide sockets, measure 96.86±6.24μm in length and 6.98±0.4μm in basal diameter. They are normally straight on the lateral margin of flagellum, especially the lateral and anterior margins of antennal club and curved on the outer margin of scape (Figures 2–A, C; Figure 3–A, D).
Table 1
Length and width of antennal segments of male and female A. tumida
Sex
|
|
Scape
(μm)
|
Pedicel
|
Flagellum (μm)
|
Total (μm)
|
|
|
(μm)
|
1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
9
|
|
Male
|
length
|
213.9±0.6
|
116.2±1.9
|
102.7±0.4
|
54.6±1.5
|
49.4±1.8
|
40.4±0.8
|
43.4 ±1.9
|
42.7 ±1.9
|
81.9±0.5
|
76.4 ±3.0
|
154.1±2.8
|
975.6±15.0
|
Female
|
length
|
162.1±1.9
|
119.5±0.8
|
102.3±0.8
|
55.5±0.3
|
57.2±0.6
|
49.5±0.5
|
52.5 ±2.5
|
49.7 ±0.5
|
84.8 ±3.7
|
84.3±0.6
|
155.5±1.4
|
972.9±8.0
|
Sensilla chaetica 2 (SC2) are long straight setae. They have sharp tips, smooth walls and scattered into tight sockets, measure 65.1±3.6μm in length and 6.5±0.3μm in basal diameter. They are inserted on pedicel and scape (Figure 3–A and D).
Sensilla chaetica 3 (SC3) are the shortest spines, located at pedicel, flagellum 9 and basal of scape. They have blunt point tips and V-shape grooved walls, and no obvious socket at base, measure 24.0±1.6μm in length and 4.6±0.3μm in basal diameter (Figure 3–A and E).
Sensilla trichodea (ST) are long and curved hairs, gradually narrower towards apex. They have sharp tips, longitudinal grooved walls and wide sockets. They are numerous and mainly distributed on antennal club, measure 17.2±0.3μm in length and 3.6±0.1μm at basal diameter (Figure 2–B and D).
Sensilla basiconica (SB) are short and cone-shaped with the most abundance on the antennae. According to the external morphological characteristics, they can be divided into 7 subtypes (Figure 2–3).
Sensilla basiconica 1 (SB1) are long-rod shaped and have the most length among all subtypes, measure 12.1±0.5μm in length and 2.6±0.1μm at basal diameter. They are straight, thin and inserted into wide sockets with tips blunt pointed and walls smooth basally and rough apically. They are only distributed in the flagellum 9 (Figure 2–B and E).
Sensilla basiconica 2 (SB2) are short and straight. They are characterized by tips sharped, walls smooth at half base and longitudinal grooved apically and sockets tight. They only distributed at terminal segment of antennal club (Figure 2–B and F).
Sensilla basiconica 3 (SB3) are corn shaped, straight or curved. They have blunt tips and alternating grooved walls, usually inserted in tight sockets. The length is 3.7±1.6μm and the basal width is 1.6±0.3μm, and only distributed on antennal club (Figure 2–B and G).
Sensilla basiconica 4 (SB4) are typical of cuticular finger-like protuberances extending tapered from base to apex. The tips are blunt point and walls are alternating grooved. They are the least antennal sensilla which inserted in wide sockets on antennal club, measure 2.1±0.1μm in length and 1.4±0.03μm at basal diameter (Figure 2–B and I).
Sensilla basiconica 5 (SB5) are ear shaped and looking like flatted SB3. They are straight or slightly curved, and inserted in wide sockets. They are 4.1±0.3μm in length and 1.99±0.08μm in basal width and all located in flagellum 9 (Figure 2–A and J).
Sensilla basiconica 6 (SB6) are upright with smooth walls and bifurcate tips. They are inserted in wide sockets on anrennifer, measured 4.3±1.6μm in length and 2.7±0.2μm in basal diameter (Figure3–B, C and F).
Sensilla styloconica (SS) are cone shaped on raised cuticle. They are divided into 2 sub-types according to their lengths.
Sensilla styloconica 1 (SS1) are short-cone shaped. They are straight with blunt point tips and smooth walls. They are inserted in wide sockets and measure 1.7±0.1μm in length and 3.8±0.1μm in basal width, and only located in middle of antennal club (Figure 2–H).
Sensilla styloconica 2 (SS2) are figure-like, they are straight or slightly curved with blunt tips and alternating grooved walls. They are inserted in wide sockets and measure 6.0±0.2μm in length and 4.3±0.1μm in basal width, and located in the anterior margin of flagellum 7 and 8 (Figure 2–A and K).
Böhm bristles (BB) are straight and short spines, clustered at anternnifer. They have sharp tips and smooth walls, and scattered into wide sockets, measure 3.9±0.2μm in length and 2.6±0.1μm at basal diameter (Figure 3–A and C).
3.2. Mouthparts
3.2.1. Gross morphology of the mouthparts
A. tumida has typical chewing mouthparts same as other insects of Coleoptera, consisting of five parts (Figure 4–B). Labrum consists of an ossified outer lip (Figure 4–A) and a soft hairy epipharynx (Figure 7–B). It forms the upper cover of the mouthpiece to prevent food from failing off, and the anterior margin of labrum is emarginated to help feeding from the edge of food. A pair of mandibles (Figure 4–C) located behind labrum, consist of apical incisor lobes for cutting and tearing food, and basal molar lobes for grinding food. A pair of maxillae located behind mandibles, consist of cardo, stipes, galea, lacinia and maxillary palpus (Figure 5–A and B). Labium is used as the posterior wall of mouthparts to hold food and consists of postmentum, prementum, paraglossa, glossa and labial palpus. Postmentum is located at base of labium and consists of submentum and mentum. Paraglossa and glossa are collectively called lingual (Figure 6–B). Hypopharynx is located in the center of preoral cavity, helping to transport and swallow food.
3.2.2. Sensilla on mouthparts
8 types of sensilla were identified on mouthparts, including sensilla chaetica (SC), 8 subtypes of sensilla basiconica (SB), 2 subtypes of sensilla placodeum (SP), 2 type of sensilla styloconica (SS), sensilla multicobular (SM), 1 type of sensilla coelocinica (Sco), 2 subtypes of campaniformia (Scam) and 1 type of Böhm bristles (BB). The division standards of sensilla on mouthparts are presented in Table S2.
Sensilla chaetica (SC) are straight spine with sharp tips and inserted in wide sockets, with longitudinal grooved walls and measured 41.0±4.6 in length and 3.3±0.3 in basal diameter. They are distributed on lateral margin of cardo, sripes, mandile and labium (Figures 4–A, C; 5–B, F and 6–B).
Sensilla basiconica 1 (SB1) are short and upright cone-shaped. They have sharp tips and smooth walls; they are inserted in wide and deep sockets, measured 4.1±0.1μm at length and 1.5±0.2μm at basal diameter. They are only distributed on maxillary palpus (Figure 5–B, H and I).
Sensilla basiconica 2 (SB2) are finger-liked. They are straight with walls smooth; tips blunt with a small pore apically. They are 3.3±0.5μm in length and 1.9±0.7μm in basal diameter, and only distributed on sensilla region of terminal segment of maxilla and labium palpus (Figures 5–G and 6–A, C).
Sensilla basiconica 3 (SB3) are straight with smooth walls and bifurcate tips. They are measured 4.7±0.2μm in length and 1.8±0.1μm in basal diameter. They are inserted in wide sockets, and only symmetrically distribute in both sides of inner margin of epipharynx (Figure 7–A, F and I).
Sensilla basiconica 4 (SB4) are small and short cone-shaped. They are straight with smooth walls and a pore at top of blunt tips. They are inserted in wide sockets and measured in 2.1±2.0μm in length and 2.5±0.2μm at basal diameter. They are distributed on the epipharynx (Figure 7–E and K).
Sensilla basiconica 5 (SB5) are hemispherical with smooth walls and a pore at top of blunt tips. They are inserted in wide and deep sockets and measured 2.7±0.1 in length and 2.9±0.1 in basal diameter. They are distributed on the epipharynx (Figure 7–B and D).
Sensilla basiconica 6 (SB6) are hemispherical. Tips blunt with a pore, walls smooth and inserted in wide sockets. They are measured in 1.9±0.1μm in length and 2.0±0.03μm in basal diameter, and only distributed on the epipharynx (Figure 7–J, K).
Sensilla basiconica 7 (SB7) are hemispherical and straight. Tips blunt with double pores, walls smooth and inserted in wide sockets. They are measured in 1.6±0.1μm in length and 2.7±0.04μm in basal diameter, and only distributed besides SB6 (Figure 7–J, K).
Sensilla basiconica 8 (SB8) are short cylindrical with constricted apex. They are inserted in a deep socket and measured 3.3±0.1 in length and 2.1±0.2 in basal diameter. They are distributed in pairs on the lateral margin of epipharynx (Figure 7–B, C).
Sesnsilla placeodeum (SP) are long stripes that grow inside tight depressions of cuticle. They are straight with smooth walls, and are divided into two subtypes according to their lengths. Sensilla placodeum 1 (SP1) are measured 25.3±0.5μm in length and gathered symmetrically at the base of the lateral margin of the terminal segment of maxilla palpus (Figure 5–B, D). Sensilla placodeum 2 (SP2) are measured 12.9μm in length and distributed singly at the lateral margin of the terminal segment of maxilla palpus (Figure 5–B, E).
Sensilla styloconica (SS) are sensory cones and inserted in elevations of the cuticle. They are medium-sized, thick and straight with smooth walls and surrounding with Sco. There are two subtypes according to the length and shape of tips.
Sensilla stylocoica 1 (SS1) are blunt at tips and measured 2.8±0.2 in length and 2.4±0.1μm in basal diameter, and concentrated in the sensilla field at tip of terminal segment of maxillary palpus (Figrue 5–G).
Sensilla stylocoica 2 (SS2) are flower-like at tips and measured 7.6±0.3μm in length and 7.6±0.3 in basal diameter, and concentrated in the sensilla field at tip of terminal segment of labium palpus (Figrue 6–A, C).
Sensilla multicobular (SM) are flower-like or bract-shaped and appear to consist of many cones that curve from base to top. They have rugose walls and no obvious sockets, measured 1.3±0.6μm in length and 2.4±0.6μm at basal diameter. They are clustered and only distributed at the end of the sensilla fields at the top of terminal labium palpus (Figures 6–A, D).
Sensilla coelocinica (SCo) are nipple-like. They are short and thick with smooth walls, tips blunt with a pore. They are 2.7±0.1μm in length and 2.9±0.9μm in basal diameter. They are inserted in the pit formed by raised cuticle, distributed along the margins of sensilla fields at the top of the terminal segment of labium and maxillary palpus, and symmetrically arranged along the middle of glossa (Figures 5–G; 6–A, B, C, E).
Sensilla campaniformia 1 (SCam1) are hemispherical concave cuticle, with small projections medially. They are symmetrically located at base of epippharynx. They are measured 3.3±0.2μm in width (Figure 7–B, G).
Sensilla campaniformia 2 (SCam2) are oval concave cuticle. They are symmetrically located at apex of epippharynx. They are measured 5.1±0.4μm in width (Figure 7–B, H).
Böhm bristles (BB) are short, straight and cone shaped. They have sharp tips and smooth walls. These sensilla measured 6.9±0.3μm in length and 3.2±0.2μm at basal diameter, all of them arranged along outer margin of lacinia (Figure 5–A, C).
Table 2
Comparison of the antennal sensilla of O. colon, M. (O.) chinensis and A. tumida.
Species
|
Feed Habit
|
Sex
|
ST
|
SC
|
SB
|
BB
|
SS
|
SCa
|
SCo
|
Total
|
O. colon
(in Cao and Huang (2016))
|
Saprophagy
(corpses of bovine)
|
Male
Female
|
3
|
2
|
3
|
1
|
1
|
1
|
\
|
6 types
11 subtypes
|
M.(O.) chinensis
(in Li (2021))
|
Phytophagy
(Host plant:
Rubus idaeus
flowers)
|
Male
|
3
|
1
|
3
|
1
|
3
|
1
|
\
|
6 types
12 subtypes
|
Female
|
3
|
2
|
3
|
1
|
3
|
1
|
1
|
7 types
14 subtypes
|
A. tumida
(in this study)
|
Saprophagy
(honey)
|
Male
Female
|
1
|
3
|
6
|
1
|
2
|
\
|
\
|
5 types
13 subtypes
|
Table 3
Comparison of the sensilla on the mouthparts of O. colon, M. (O.) chinensis and A. tumida.
Species
|
Feed Habit
|
Sex
|
ST
|
SC
|
SB
|
BB
|
SP
|
SCo
|
SCam
|
SM
|
SS
|
Total
|
O. colon
(in Cao and Huang (2016))
|
Saprophagy
(corpses of bovine)
|
Male
Female
|
1
|
2
|
7
|
1
|
2
|
1
|
2
|
\
|
\
|
7 types
16 subtypes
|
M.(O.) chinensis
(in Li (2021))
|
Phytophagy
(Host plant:
Rubus idaeus
flowers)
|
Male
|
2
|
2
|
7
|
1
|
2
|
2
|
1
|
\
|
\
|
7 types
17 subtypes
|
Female
|
2
|
2
|
6
|
1
|
2
|
2
|
1
|
\
|
\
|
7 types
16 subtypes
|
A. tumida
(in this study)
|
Saprophagy
(honey)
|
Male
Female
|
\
|
1
|
8
|
1
|
2
|
1
|
2
|
1
|
2
|
8 types
18 subtypes
|