Holotype ♂. Arizona: Santa Cruz Co., Peña Blanca Canyon, 4000', 22–23 Jul 1999, J. B. Walsh; CNCLEP 79872 [CNC]. Paratypes 7♂ 5♀. Arizona: Santa Cruz Co., 4 mi W Peña Blanca Lake, 20 Jul 2000, R.B. Nagle, 1♂; CNC dissection # 17027, [CNC]; Santa Cruz Co., Hall Ranch, 31.60°N 110.73°W, [7 km NNE Patagonia], 27 Aug 2016, R. B. Nagle 1♀, CNC dissection # CNC17658, DNA voucher # 16-132 [CNC]; same data as previous, 1♀, dissection # RBN001, DNA voucher # 16-133 [RBN]; same data as previous, 1♂, [RBN]; Santa Cruz Co., Patagonia roadside rest stop, mile marker 15.6, 5.Jul.2004, B. Walsh, 1♀ [RBN]; Santa Cruz Co., California Gulch, 31°25'18.3"N, 111°14'40.02"W, 21.Jul.2012, E. J. Rand, 8♂ 2♀, [EJR].

Euchaetes nancyae is named in honor of the senior author’s wife who not only has long supported her husband’s study of southwest moths but has also played host to numerous visiting lepidopterists. Nancy won a weekend trip to the Hall Ranch, which led to the initial capture and discovery of this species.

Euchaetes nancyae is superficially similar to other species of Euchaetes and Pygarctia that have predominantly white or grey-white wings, but can generally be distinguished by examination of the head and thoracic colour patterns alone, as illustrated in Figs 1–6. Euchaetes helena and Pygarctia flavidorsalis Barnes & McDunnough have yellow instead of pink scaling on the head. The most common look-alike species that is sympatric with E. nancyae is Pygarctia roseicapitis (Neumögen & Dyar), but E. nancyae differs most obviously from that species by the white (Figure 1) rather than pink (Figure 4) vertex of the head. Wing and thoracic colour of E. nancyae is also similar to those of E. castalla Barnes & McDunnough; however, E. castalla has less pink scaling along the posterior margin of the head that is interrupted medially with white; the pink scales are often not visible without magnification and give the prothoracic collar the appearance of being entirely white (Figure 3); females of E. castalla have a white abdominal tuft, whereas the tuft is absent in E. nancyae. The distributions of E. nancyae and E. castalla overlap in at least southeastern Arizona (e.g. California Gulch, Pima Co.), and may do so also in northern Mexico. Morphologically, the male uncus and vesica of E. nancyae is highly distinctive, particularly the setation of the lateral lobes and the structure of the dorsomedial lobe, as described below and illustrated in Figs 9, 10. Females differ from those of E. helena in the shape of the cervix bursae and the papillae anales (Figs 11, 12). Structurally, E. nancyae differs from all Pygarctia species by the lack of a foretibial spine and a shorter 2^nd segment of the labial palpus, in addition to the genus traits given below in the Discussion.

DNA barcode sequences of E. nancyae are more than 6% divergent from all other North American Euchaetes, in addition to several undescribed Mexican and Central American species. The two sequenced specimens of E. nancyae differ by 0.46%, forming a unique BIN (Barcode Index Number; Rathnasingham and Hebert 2013). The most similar species based on nearest-neighbour distance analysis is Euchaetes helena, differing by a minimum of 6.61%.

Figures 1–6. 

Head and thoracic colour patterns of Euchaetes nancyae and similar species. 1 Euchaetes nancyae 2 Euchaetes helena 3 Euchaetes castalla 4 Pygarctia roseicapitis 5 Pygarctia flavidorsalis 6 Pygarctia lorula.

Head. Vestiture of sexes similar; frons and vertex covered with white scales; posterior margin of head with a delicate ring or collar of pale orange-pink scales, extending around posterior and ventral margin of eye; male antenna bipectinate, longest rami 2.5 x longer than antennal segment, scape and proximal third dorsally scaled with white, grading into darker pale grey-brown scaling over distal two-thirds; female antenna biserrate, dorsal vestiture similar to that of male. First segment of labial palpus with relatively long, pale orange-pink scales along ventral edge, second segment covered with white scales, third segment covered with light brown scales; length ratio of segments 1:2:3 = 2.2 : 3.0 : 1.0.

Thorax. Vestiture of sexes similar; patagium, tegula, and thorax covered with greyish-white scales, lateral margin of patagium and tegula with a border of long pale pink hairs extending underneath wings; foreleg coxa pale orange pink with a medial patch of pale grey-brown scales; foretibia without claw at apex; femur and tibia white dorsally, ventrally a mix of white and pale grey-brown scales, tibial spurs and tarsal segments pale grey brown; mid and hind legs with coxa, femur, and tibia covered with a mix of white and pale grey brown scales; metepisternum lacking microtymbals.

Figures 7, 8. 

Adults of Euchaetes nancyae male (7) and female (8).

Forewing and hindwing. Dorsally both wings and fringe entirely silvery greyish white, lacking any distinguishable markings in both sexes; ventrally, forewing a slightly darker shade of greyish white than hindwing. Male forewing length 15.2–15.8 mm (n = 2), female 18.6–19.5 mm (n = 2).

Abdomen. Colouration of sexes similar; dorsum pale silvery pinkish orange with a row of 6–7 dorsomedial macules at anterior tergal margin, this dark grey and indistinctly ringed with whitish grey; laterally with a row of prominent black maculae at anterolateral angle; ventrum silvery whitish grey; female without terminal tuft of long scales; intersegmental membrane between sternites 7–8 of male with well-developed coremata, nearly length of abdomen when fully extended.

Male genitalia. Gross morphology typical of Euchaetes, with costa and sacullus deeply divided, costa forming a long prong-like finger, and sacculus forming a membranous, elongate lobe; saccus broad and collar like, not tapered to a point medio-ventrally as in many arctiines; juxta lightly sclerotized and indistinctly differentiated, broadly U-shaped; uncus complex, with a dorsoventrally flattened lateral lobe; each lobe densely setose, forming an evenly sinuate “hair-do” that terminates in an anteriorly-directed, dense, pointed tuft; dorsomedial process of uncus with a pyramidal base, terminating in a laterally flattened crest; apex of uncus laterally flattened and shaped like a broad bird’s beak in lateral view; phallus 5 × longer than wide, curved slightly dorsad; vesica roughly kidney shaped with small subbasal diverticulum; disto-medial row of approximately 20 sawtooth-like cornuti, decreasing in size towards ductus ejaculatorius.

Female genitalia. Distal margin of papillae anales irregular rounded quadrate; papillae anales moderately setose, densely and finely setose dorsally below apertures of dorsal pheromone gland; pseudopapillae anales membranous; dorsal pheromone gland unbranched, terminating as pair of openings; length of anterior and posterior apophyses approximately equal to maximum width of papillae; cervix bursae moderately sclerotized, internally rugose near junction with ductus, remainder with regularly spaced microtrichia or spicules; ductus bursa flattened dorsoventrally and ribbon-like; corpus bursa essentially spherical, with two star-like signa, one dorsal and one ventral; ventral signum with four arms or rays, dorsal one with three.

Figures 9, 10. 

Male genitalia of 9 Euchaetes nancyae and 10 E. helena.

The larval hostplants of E. nancyae are unknown. Two females oviposited approximately 30 shiny yellow-white eggs measuring 0.8 mm in diameter. Euchaetes nancyae lacks female abdominal tufts, and eggs were not covered with abdominal setae. Presumably, all Euchaetes species that possess female abdominal tufts cover their eggs, such as E. egle (Drury). The first instar larvae were offered several species of Asclepiadaceae including Funastrum cynanchoides (Decne.) Schltr. and one species of Euphorbiaceae, Chamaesyce hyssopifolia (L.), but refused to feed and perished. The related species Euchaetes zella and E. perlevis feed on Funastrum cynanchoides, and Euchaetes fusca larvae feed on the Cnidoscolus angustidens Torr. (Euphorbiaceae). The larval foodplant of the probable sister species Euchaetes helena from Texas is also unknown, but larvae of that species also do not accept Asclepiadaceae (D. Wagner, pers. comm.).

Euchaetes nancyae is known only from the type series, collected in Sonoran desert habitat along the Mexican border in Santa Cruz County, Arizona. It undoubtedly also occurs in adjacent parts of Mexico. Euchaetes helena occurs in Texas (Davis Mountains), and was listed as likely occurring in Mexico by Vincent and Laguerre (2014); we here confirm the occurrence of E. helena from Coahuila, verified by DNA barcode sequence (DNA vouchers CNCLEP 00113425 and CNCLEP00113424).

Figures 11, 12. 

Female genitalia of 11 Euchaetes nancyae and 12 E. helena.