1172


Journal of Paleontology 91(6):1166–1177


Figure 5. Triassonepa solensis n. gen. n. sp. growth series: (1) Instar I, VMNH 94575a; (2) Instar II, VMNH 50512; (3) Instar III, VMNH 53871; (4) Instar IV, VMNH 54155; (5) Instar V, VMNH 52498. Scale bar is 2mm, located in photo 1.


Because the apical parts of modern heteropteran hemelytra are thin and membranous, it is unlikely that these regions would have been preserved. In addition, the abdominal spiracles of T. solensis are located on its dorsal surface, which is typical of insects that breathe underwater via a plastron (a thin film of air held beneath the wings and used as a physical gill). In order for T. solensis to use a plastron, as is done by many modern aquatic insects including belostomatids (e.g., Abedus herberti Hildago, 1935; see Goforth and Smith, 2012), its hemelytra would need to fully cover its spiracles (i.e., to the edges of the abdomen). The 8th abdominal segment of Triassonepa solensis n. gen.


n. sp. contains two lateral, paddle-shaped lobes and two medial processes resembling respiratory tubes (Fig. 4.11). It is morphologically quite similar to the eighth abdominal segment of female naucorids (particularly Ilyocoris exclamationis Scott, 1874 as illustrated by Lee, 1991). This suggests that: (1) T. solensis occupies a very basal position within Belostomatidae, and (2) the majority of the preserved specimens were likely female. However, two specimens (VMNH 94671, Fig. 4.12, holotype; VMNH 94672, Fig. 2.4) possess terminalia with a slightly different structure. The 8th abdominal segments of these specimens are more elongate and possess two apical, articulated appendages that resemble claspers. It is therefore likely that these two specimens are males. Immature Triassonepa solensis n. gen. n. sp. were


separated into instars based on total body lengths (Fig. 5). Because the head was not often preserved, there is a minor


amount of uncertainty in some of the measurements. A total of 51 nymphs were measured from the anterior margin of the head to the apex of the abdomen, yielding five size classes (Fig. 6): Instar I, 1.6–2.3mm (mean 2.0mm); Instar II, 2.6–3.4mm (mean 3.0mm); Instar III, 4.0–5.2mm (mean 4.6 mm); Instar IV, 6.2–7.7mm (mean 6.9 mm); Instar V, 9.5–10.3mm (mean 9.9mm). Each instar was ~1.5x larger than the preceding one. Modern belostomatids also have five instars and show a similar growth ratio of 1.2–1.5 with each successive instar (Tables 2, 3). These ratios correspond to Dyar’s Rule, which states that an insect instar is ~1.4x the size of its previous instar.


Discussion


Belostomatid habitats.—Due to the presence of swimming fringes on the hind legs of Triassonepa solensis n. gen. n. sp., it is reasonable to assume that this species had similar physio- logy and behaviors to modern belostomatids. Understanding the habitats of these modern belostomatids has implications for determining the nature and chemistry of ancient ‘Lake Solite.’ Belostomatids live in a wide variety of habitats, but are


most commonly found in shallow bodies of water with marginal vegetation. Kashian and Burton (2000) reported Lethocerus sp. from the wetlands of northern Lake Huron in areas dominated by sedges. Belostomatids also occur in many of India’s small freshwater lakes. Majumder et al. (2013), for example, found two genera (Lethocerus and Diplonychus) living in the marginal


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