Dactylosternum cacti (LeConte, 1855)

Archangelsky, Miguel, Rodriguez, Georgina, Torres, Patricia L. M., Liesa, Ciemep-Conicet, Dbbe & FCEyN, 2016, Primary chaetotaxy and larval morphometry of Phaenonotum exstriatum and Dactylosternum cacti (Coleoptera: Hydrophilidae), Acta Entomologica Musei Nationalis Pragae 56 (1), pp. 167-193 : 169-181

Dactylosternum cacti (LeConte, 1855)

Examined larvae. Nine 1 st instar larvae (L1), three 2 nd instar larvae (L2) and 6 3 rd instar larvae (L3) reared from the adults collected in the following localities: USA: ARIZONA: Maricopa Co., 29.vii.1995, M. Archangelsky lgt. GUATEMALA: El Progreso, 5 km W junction CA14 and CA9, 7.viii.1991, T. K. Philips and P. W. Kovarik lgt. Adults and larvae were reared in laboratory and the complete life cycle was obtained (ARCHANGELSKY 1994).

First instar larva (Figs 7, 11–22). Diagnosis. Head capsule suboval; frontal lines inversely bell-shaped, merging before occipital foramen, coronal line present, extremely short ( Fig. 11 ); clypeolabrum asymmetrical, nasale slightly asymmetrical, shorter than lateral lobes ( Figs 11 , 13 ); lateral lobes of epistome asymmetrical, projecting farther than nasale, left lobe larger than right lobe, slightly serrated on inner margin ( Fig. 11 ); posterior tentorial grooves close to midline, before basal half ( Fig. 12 ). Cervical sclerites present ( Fig. 11 ). Antenna short ( Fig. 14 ), basal antennomere the longest, sensory appendage 0.55–0.63× as long as third antennomere. Mandibles asymmetrical ( Figs 15, 16 ), right one slightly smaller ( Fig. 16 ), with a stout inner tooth serrated distally; left mandible with two sharp inner teeth on basal half, covered by a short pubescence ( Fig. 15 ); both mandibles serrated on distal inner margin. Maxilla ( Figs 17, 18 ) with large stipes, longer than palpus, inner margin with dense pubescence (formed by fine spinulae) on inner and outer margins; first palpomere the longest, wider than long, dorsally pubescent; second and third palpomeres shorter than both first and fourth palpomeres. Labium with submentum completely fused to head capsule, lines of fusion not evident, mentum slightly wider than prementum ( Fig. 12 ); basal palpomere the shortest; ligula reduced ( Figs 19, 20 ); dorsally with sharp cuticular spines on membrane between mentum and prementum, and between prementum and first palpomere ( Fig. 19 ). Hypopharynx asymmetrical, membranous and pubescent, developed as a lobe on left side (Fig. 9, third instar larva). Pronotal plate large, covering most of pronotum; those of meso- and metathorax shorter. First abdominal tergum membranous (see Fig. 1, third instar larva); abdominal segments one to seven with two dorsal rows of spines; dorsal plate on abdominal segment eight large, suboval; abdominal segments one to eight with two horizontal rows of strong spines on ventral face, laterally ending in lobed projections or “prolegs” which bear larger spines (Figs 2, 3,10 show the character in third instar larva); legs short, five-segmented (Figs 3, 21, 22).

Chaetotaxy: Two innermost seta of gFR1 short and stout, placed on small lobe (Figs 7, 13); PA26+27+28 not closely aggregated ( Fig. 12 ); seta AN9 absent ( Fig. 14 ); inner margin of stipes with row of additional unifid setae (gMX2) ( Fig. 17 ); pore LA8 located in a distal position ( Fig. 19 ); with additional seta near LA10 ( Fig. 19 ); sensilla LA11 and LA12 absent ( Figs 19, 20 ). The illustrated specimen ( Fig. 11 ) has an extra seta (PA?) near PA13, this seta is not present on the left side nor in any other examined specimens, therefore it is not considered an additional seta.

Primary chaetotaxy ( Figs 11–22 ). Head capsule ( Figs 11–13 ). Frontale with 30 sensilla: two very short setae on basal third, close to frontal lines (FR1); two pores (FR2) and two minute setae (FR3) close to midline on distal half; five pairs of setae (FR5 rather long, FR6 long, FR7 short, FR9 rather long, FR10 long) and two pairs of pores (FR4, FR14) close to base of each antenna; distal area of frontale with two pairs of setae (FR8 long, FR12 short) and three pairs of pores (FR11, FR13, FR15); nasale with six short setae (gFR1), middle two stouter and placed on a small lobe (Figs 7, 13), each epistomal lobe with two short anterior setae (gFR2). Each parietale with 31 sensilla. Dorsal surface with a basal longitudinal row of four very short setae (PA1, PA2, PA4, PA5) and one pore (PA3); one subbasal pore (PA6) close to frontal lines; three long setae (PA12, PA13, PA14) and one very short seta (PA7) arranged in a transverse row posterior to midlength; three long setae

◄Figs 1–10. 1–3 – habitus of the third instar larva of Dactylosternum cacti (LeConte, 1855) (1 – dorsal view; 2 – ventral view; 3 – lateroventral view). 4–6 – habitus of the third instar larva of Phaenonotum exstriatum (Say, 1835) (4 – dorsal view; 5 – ventral view; 6 – lateral view). 7 – middle setae of gFR1 of Dactylosternum cacti ; 8 – branched setae on head capsule of Phaenonotum exstriatum (second instar larva); 9 – labium with hypopharyngeal lobe of D. cacti (third instar larva, ventral view); 10 – rows of spines and ‘proleg’ of D. cacti (third instar larva, ventral view). Scale bars: Figs 1–6 = 1 mm, Fig. 9 = 0.05 mm.

(PA8, PA11 and PA14) and two pores (PA10, PA15) in a transverse row in the area surrounding the stemmata; three setae (PA9 long, PA20 short, PA21 long) and one pore (PA19) on anterolateral corner of head capsule. Ventral surface with three pores (PA23, PA24, PA25) and one rather long seta (PA22) on anterolateral corner, close to mandibular acetabulum; two long setae (PA16, PA18) and two pores (PA17, PA30) along outer margin; one short seta (PA26), one long seta (PA28), and two pores (PA27, PA29) forming a longitudinal row closer to midline.

Antenna ( Fig. 14 ). A1 bare, with five pores, three dorsal on distal half (AN1 at midlength on outer margin, AN2 close to midline, AN4 distally on inner margin) and two ventral, on distal margin (AN3, AN5). A2 with one dorsal pore (AN6) on distal third, two short subapical setae (AN7 on outer margin, AN8 close to midline) close to base of SE1, and two subapical setae on inner margin (AN10 long, AN11 short); AN9 absent. A3 with a group of at least three short setae, three long setae and one pore (gAN).

Mandibles ( Figs 15–16 ). Both mandibles with six sensilla: one short seta on outer margin at midlength (MN1), one minute seta on outer margin at distal quarter (MN5), three dorsal pores at level of inner retinaculum (MN2, MN3, MN4); MN6 minute, difficult to see, located subapically close to inner margin.

Maxilla ( Figs 17–18 ). Cardo with one rather long seta ( MX 1). Stipes with a row of ca. 16–17 stout and short setae along inner margin (gMX2), those at midlength closely aggregated, basal seta of that row likely homologous to MX 7; ventral face of stipes with three pores, one at midlength ( MX 2), one subapical close to inner margin ( MX 3) and one subapical on outer margin ( MX 4); two long setae distal to pore MX 4 ( MX 5, MX 6). MP1 dorsally with one subbasal spiniform seta ( MX 16) on inner margin and one pore close to base of appendage ( MX 17); ventrally with two long subapical setae ( MX 13, MX 14) and two pores ( MX 12 on outer margin, MX 15 at base of appendage); inner appendage with three long setae and one short sensillum (gAPP). MP2 with two pores, one ventral and apical ( MX 18) and one dorsal on membrane connecting with MP3 ( MX 19); subbasal minute seta present on outer margin ( MX 27). MP3 with two long setae and two pores, one seta dorsal, on outer margin ( MX 23), the other ventral on mesal margin ( MX 21); both pores on ventral surface, one close to seta MX 21 ( MX 22), the other apical, close to outer margin ( MX 20). MP4 with one long basal seta dorsally ( MX 24) on inner margin and two subapical pores on outer margin ( MX 25 digitiform and dorsal, MX 26 ventral); a group of at least six or seven short setae constitute gMX.

Labium ( Figs 12 , 19–20 ). Submentum with two pairs of setae on anterior margin ( Fig. 12 ), one long (LA1), the other minute (LA2). Mentum with two rather long setae (LA3) and two pores (LA4) on ventral surface close to anterolateral angle, LA3 anterior to LA4. Prementum ventrally with two pairs of setae (LA5 minute and basal, LA6 long and subapical) and one pair of subapical pores (LA7); dorsally with one apical pair of pores (LA8), one apical pair of minute seta-like sensilla (LA9) and two pairs of stout setae distally on membrane connecting with labial palpi, inner pair very long (LA10), outer pair additional (LA■). Sensilla associated with ligula absent (LA11, LA12). LP1 with one minute seta (LA13, ventral) and one distal pore (LA14, dorsal) on membrane connecting with LP2; LP2 with one subapical pore on outer face (LA15) and a group of at least five setae and two pores in the apex constitute (gLA), four setae short, one rather long.

Legs ( Figs 21–22 ). The number and position of pores are the same in pro-, meso-, and metathoracic legs. Coxa with 3 pores (1 anterodorsal, 2 posterior) and 17 setae (12 anterior, 5 posterior); trochanter with 8 pores (5 anterior, 3 posterior) and 6 setae (3 anterior, 3 posterior); femur with 2 pores (1 anterior, 1 posterior) and 7 setae (5 anterior, 2 posterior); tibiotarsus with 1 anterior pore and 16 setae (10 anterior, 6 posterior); pretarsus with two subbasal setae (1 anterior, 1 posterior).

Morphometry. See Table 2.

Second instar larva. Chaetotaxy. More similar to the third instar than to the first. Head capsule. Frontale and parietale bearing several small secondary sensilla; middle setae of gFR1 not stout, lobe absent. Antenna. Without secondary sensilla; sensory appendage proportionally longer than in third instar. Mandible. With several minute secondary sensilla on outer face. Maxilla. With two or three minute secondary setae on ventral face of stipes, gMX2 with 17–18 setae, distal ones slightly longer and narrower than basal ones. Labium. Without secondary sensilla. Number and distribution of pores and setae in legs similar to first instar larva.

Morphometry. See Table 2.

Third instar larva ( Figs 23–32 ). Chaetotaxy. Head capsule ( Figs 23–25 ). Frontale with eight to 10 secondary sensilla arranged in a row subparallel to each frontal line starting behind FR1 and extending slightly past pore FR2; a minute secondary seta present in some specimens between FR3 and FR6 ( Fig. 25 ); middle setae of gFR1 not stout, lobe absent. Each parietale with 15 to 17 short secondary setae and one or two small secondary pores on dorsal surface; ventrally with 17 to 18 short or minute secondary setae and one secondary pore in some specimens. Posterior tentorial grooves at midlength, slightly more distally placed than in first instar larvae. Antenna ( Fig. 26 ) without secondary sensilla, sensory appendage shorter, 0.42–0.50× as long as third antennomere. Mandibles ( Figs 27–28 ). Each mandible with eight to nine short secondary setae, most of them on basal outer face. Maxilla ( Figs 29–30 ). Stipes with three secondary setae on ventral face, one very short close to outer margin, one minute at midlength between MX 3 and MX 4, and one short on outer margin close to MX 5 and MX 6; inner row of setae on stipes with ca. 18–19 setae (gMX2), distal ones longer and more slender, basal one slightly apart from others, probably homologous to MX 7. Labium ( Figs 31–32 ) without secondary sensilla. Number and distribution of pores and setae in legs similar to first instar larva.

Morphometry. See Table 2.

Figures

Figs 11–12.Dactylosternum cacti (LeConte, 1855), chaetotaxy of the head capsule of the first instar larva. 11 – dorsal view; 12 – ventral view. Scale bar = 0.1 mm.

Figs 13–16. Dactylosternum cacti (LeConte, 1855), chaetotaxy of first instar larva. 13 – detail of clypeolabrum; 14 – left antenna, dorsal view; 15 – left mandible, dorsal view; 16 – right mandible, dorsal view. Scale bars: Fig. 13 = 0.1 mm, Figs 14–16 = 0.05 mm.

Figs 17–20. Dactylosternum cacti (LeConte, 1855), chaetotaxy of first instar larva. 17 – left maxilla, dorsal view; 18 – maxilla, ventral view; 19 – labium, ventral view; 20 – labium, dorsal view. Scale bars = 0.05 mm.

Figs 21–22. Dactylosternum cacti (LeConte, 1855), prothoracic leg of first instar larva. 21 – anterior view; 22 – posterior view. Scale bar = 0.05 mm.

Figs 23–24.Dactylosternum cacti (LeConte, 1855), chaetotaxy of the head capsule of the third instar larva. 23 – dorsal view; 24 – ventral view. Scale bar = 0.5 mm.

Figs 25–28. Dactylosternum cacti (LeConte, 1855), chaetotaxy of third instar larva. 25 – detail of clypeolabrum; 26 – left antenna, dorsal view; 27 – left mandible, dorsal view; 28 – right mandible, dorsal view.Scale bars = 0.1 mm.

Figs 29–32. Dactylosternum cacti (LeConte, 1855), chaetotaxy of third instar larva. 29 – maxilla, dorsal view; 30 – maxilla, ventral view; 31 – labium, dorsal view; 32 – labium, ventral view. Scale bars: Figs 29–30 = 0.1 mm, Figs 31–32 = 0.05 mm.