Most neuropterans have only three larval stages; i.e., the third is the last one. After cocooning, we labelled the specimens by day of collection: Day 1 to Day 12 until the adults emerge. The cranial musculoskeletal system and the cephalic nervous system of the third instar larvae (including the prepupal phase, from Day 1 to Day 4), the pupae (from Day 5 to Day 12), and the adults of Chrysopa pallens were reconstructed and described. In the skeletal system of the larval head, we focused more on the outer cuticle. For the pupal head, we focus on the inner cuticle.
The external structures of head are described. For the prepupae (i.e. specimens from Day 1 to Day 4), the skeletal system is almost identical to what is found in the 3rd instar larvae, thus only the latter is described in detail. At Day 5, the larval cuticle cracked, and the pupa develops gradually during the following 7 days. We here only describe the well-developed pupae as found on the 11th day. The transformation of muscles inside the cocoon are also described. The description of head muscles in the 3rd larva, Day 11 pupa, and adult (♂) is also presented in Tables 1-3, respectively.
Third instar larvae (Fig 1A). Body of living third instar larvae fusiform and humped. Length ~7.00 mm and height ~1.30 mm. Cuticle light brown with dark brown markings dorsolaterally. Spinules and long microsetae present dorsally. All setae smooth, dark brown to light brown. Head dorsoventrally flattened 0.70 mm in length and 1.00 mm in width, with strongly prognathous prominent sucking stylets. Thorax unsclerotized with rows of short, acute setae. Legs slender and well developed, inserted on semi-membranous ventrolateral articulatory areas posteriorly. Lateral tubercles broadly short cylindrical dorsolaterally and tapering distally with elongated setae. Long setae all tapering and hooking at tips. Tubercles and long setae carry the debris for camouflage.
Prepupae (Fig 2). FromDay 1 to Day 4: Prepupae immobile adecticous exarate type. Cuticle light brown-yellow. Body C-shape with 5.0 mm in length and 3.0 mm in width. Head bends inward, morphologically almost same to larvae. Segments of thorax and abdomen similar in shape. Lateral tubercles smaller and long setae disappeared. Cocoon 4.0 mm in length and 3.0 mm in width, with dead aphids covering the cocoon (Fig 1D).
Pupae (Figs 1B, 2-3). FromDay 5 to Day 10: Color and body shape remains unchanged. At Day 5, larval cuticle cracked and wings present. Larval cuticle gathers under abdomen in cocoon (Fig 1E). During pupal stage, two layers of cuticle are apparent, the very thin and transparent outer one is the pupal cuticle, and the inner one is the adult cuticle. Pharate adult 6.00 mm in length and 2.50 mm in height. Head 1.5 mm wide and 1.2 mm long. The head is very different from that of the larvae due to the hypognathous mouth parts. Compound eyes, scape, labrum, and mandible similar to adults. Color of labrum and mandibles turn red to crimson from Day 6 to Day 10. Compound eyes red to metallic black-red. Maxillary palps, labial palps, and curly antenna present with milky color. Frontoclypeal sulcus present. Wings become larger in size and folded in wing sheath. Prothorax, mesothorax, metathorax, and legs similar to adults in shape. Short setae present on frons. Day 11: Pharate adults develop well within pupal sheath, less sclerotized than adults. Wings brown to dark from base to distal margin distinctly. Day 12: Pupae break out from cocoon (Fig 1F). After 3 hours, they emerge (Fig 1G).
Adults (Fig 1C). All structures well-developed, pale yellow. Adults 12.00 mm in length and 4.00 mm in height. Head 2.00 mm in width and 1.50 mm in length.
Third instar larvae (Fig 4: A-C). Head prognathous, roughly triangular, round posteriorly. Dorsum cream to light yellow with dark brown markings. Frontal markings confluent mesally, elongate. Epicranial markings paired, V-shape, not confluent mesally, extending to cervical margin. Eyes with six stemmata. Clypeus and labrum unmarked, continuous with frons. Membranous connection between labrum and clypeus completely reduced. Frontoclypeal sulcus absent. Anterior margin of head oblique in lateral view. Front region V-shape posteriorly and parallel-side anteriorly. Mandible amber, dark apically. Ventral maxilla smooth. The sucking tubes formed laterally by the interlocking of the mandibles and maxillae. Labium light brown. Gula absent.
Pupae (Fig 4: D-E). Day 11: Pharate adult head hypognathous, nearly triangular in frontal view, yellow to pale brown from vertex to mouthparts. Posterior vertex slightly concave. Compound eyes hemispherical, metallic black, occupying half of head width. Ocellus absent. Antennas locate between compound eyes. Antennomeres curly and almost one and a half times the body length, covering on sides of body. Clypeus broad. An indistinct suture present between clypeus and labrum. Lateral gena strongly round. Ventrally, labium connects with maxilla, which possesses 5-segments palpus.
Adults (Fig 4: F-H). Same shape and color to Day 11 pupae. Posterior vertex concave. Compound eyes large and metallic black, composed of numerous small and hexagonal ommatidia. Ocellus absent. Scapus swollen in antennal socket. Antenna filiform and almost as long as body length. Head nearly wedge-shaped in lateral view, gradually narrowing to mouthparts. Ecdysial line vestigial. Frontoclypeal sulcus and frontogenal suture present. Dorsolateral longitudinal furrow extends from dorsolateral margin of hind head capsule to mandible articulation. Lateral occipital lobes slightly exposed and hemispherical. Frontogenal suture connects anterior antennal fossa with dark anterior tentorial pits. Subgenal suture above mandible articulation vestigial. Lateral clypeus round. Anterior clypeus concave slightly with convex median line.
Third instar larvae (Figs 5: larva in A, 6). Tentorium fully sclerotized, tubular, solid throughout, connecting anterior tentorial pits at posterolateral clypeal margin with posterior tentorial pits at the foramen magnum. Tentorial bridge (tb) connects posterior tentorial arms (pta). Anterior tentorial arms (ata) diverge slightly. Dorsal tentorial arms (dta) well developed, attaching to head capsule directly.
Prepupae (Fig 6). At Day 1, ata, pta, and tb still exist, but tentorium dramatically compressed. By Day 2, tentorium disappeared. From Day 3, new tentorium present, including two separated arms.
Pupae (Figs 5: pupa in A, 6). Tentorium develop gradually at the following days. Boundary of ata and pta indistinct before Day 6. By Day 10, tb present. By Day 11, Tentorium sclerotized and hollow. Laminatentorium (lt) present which serves as attachment area of muscles (0an1, 0mx3, 0mx4, and 0mx5). Ata slender and diverge anteriorly.
Adults (Figs 5: adult in A, 6). Tentorium fully sclerotized, connecting larger anterior pits at posterolateral clypeal margin with posterior pits below occipital. Dta present, but very thin. Lt protruding, serving as the attachment of 0an1, 0mx3, and 0mx4.
Third instar larvae (Fig 4: A-C). Labrum fused to clypeus but recognizable by slightly convex structure. Musculature: in Fig 5: larvae in B.
Pupae (Fig 4: D-E). Day 11: Pharate adult labrum dark brown and clypeus brown. Anterior labrum margin slightly convex. Anterolateral edges round. Musculature: in Fig 5: pupa in B.
Adults (Fig 4: G-H). Labrum short, moving freely by labrum muscles. Anterior margin slightly convex. Two short tormae present on posterolateral labrum. Musculature: in Fig 5: adult in B.
Third instar larvae (Fig 4: A-C).Antenna glabrous and multisegmented in a slightly elevated socket. Basal segment globular and tapering distally. Pseudosegments cylindrical and separated indistinctly. Apical antennomere slender. Musculature: in Fig 5: larva in B.
Pupae (Fig 4: D-E). Day 11: Pharate adult antennae filiform and multisegmented, composed of a scapus, pedicellus and flagellomeres. Flagellomeres extremely elongate, about 1.5 times as long as pupal length, covering sides of thorax. Scapus proximally wide and narrow distally. Pedicellus nearly cylindrical with almost identical diameter and length. Musculature: in Fig 5: pupa in B.
Adults (Fig 4: G-H). Antenna filiform, about 1/3 as long as forewing. Same location to pharate adult. Socket indistinct. Short setae present around each flagellomeres. Musculature: in Fig 5: adult in B.
Third instar larvae (Fig 4: A-C). Mandibles strongly elongate, slender with apical parts, slightly upturned, longer than labial palps, closely connected with elongate maxilla. Sucking channel enclosed by mandible and maxilla. Basal mandible wide. Apical mandibular stylet curved mesad and apically pointed. Mola, prostheca and subapical teeth absent. Mandibular surface smooth. Musculature: in Fig 5: larva in C.
Pupae (Figs 4: D-E, 7: pupa). Day 11：Pharate adult mandibles roughly triangular and not quite symmetric. Joints not clear. Upper surface convex and ventral concave. Both left and right mandibles possess three apical incisors. Molar process presents in middle region of mesal edge. Ventromesally, left molar concave to fit with convex right one. Musculature: in Fig 5: pupa in C.
Adults (Figs 4: G-H, 7: adult). Mandibles heavily sclerotized. Primary mandibular joint is a globular protrusion, articulated with shallow emargination of head capsule. Secondary mandibular joint formed by a cavity of mandible and a corresponding protrusion of head capsule. Left and right mandibles moderately asymmetric. Each has an apical incisor. Dorsal side slightly convex and ventral side moderately concave. Cutting edge nearly straight on left mandible but curved on right. Small triangular molar process present in middle region of mesal edge. It is more distinct on right than on left. Musculature: in Fig 5: adult in C.
Third instar larvae (Fig 4: A-C). Maxilla composed of a proximal element, an intermediate part and an elongate distal maxillary stylet. Proximal element small, round laterally, oblique anteriorly. Intermediate piece larger, round laterally. A seta inserted in median region. Maxillary stylet elongates, similar to mandible in shape, forming the ventral part of sucking jaw. Apical part enfolds mandible. Musculature: in Figs 5: larva in D, 8: larva in A.
Pupae (Fig 4: D-E). Day 11: Pharate adult maxilla posterior to mandible. Cardo roughly quadrangular, broad. Stipes in similar shape with cardo and narrowing distally. 5-segments maxilla palpus insert on stipes distolaterally. Palpomere 1 shorter and broader. Palpomere 2 longer than 1 but wide distally. Three distal palpomeres slender. Palpomere 5 with a spindle-shaped apex. Proximal lacinia fused to dorsal stipes. Distal part slightly sickle-shaped. Galea slender proximally and wide distally, inserting between palp and lacinia. Musculature: in Figs 5: pupa in D, 8: pupa in A.
Adults (Fig 4: G-H). Maxilla connects with submentum by membrane. Cardo roughly triangular. Stipes narrower and longer than cardo, forming an acute angle laterally at base. Palp inserts in lateral stipes. Palpomere 1 much broader than other 4 segments. Distal three palpomeres elongate and slender. Lacinia basally fused to dorsal stipes. Distal part slightly curved and sickle-shaped. Galea includes slender basigalea and broader distigalea. Musculature: in Figs 5: adult in D, 8: adult in A.
Third instar larvae (Fig 4: A-C). Labium composed by submentum, mentum, and prementum, forming a complex with anterior hypopharynx. Submentum narrow and rectangular, laterally connecting with cardo. Anterior edge separated from mentum by distinct convex. Anterior mentum flat, wide, round anterolaterally. Two pairs of setae insert at anterior mentum. Prementum small and medially divided by a cleft. Glossae, paraglossae, and ligula absent. 3-segments palp (lap) distinctly elongate. Basal segment cylindrical. Palpomere 2 extremely elongate, about ten times as long as wide and slightly wide distally. Palpomere 3 slender, with same length to palpomere 1. Musculature: in Fig 8: larva in B.
Pupae (Fig 4: D-E). Day 11: Pharate adult submentum short and narrow, separated by mentum by suddenly wide anterior margin. Mentum flat and slightly swollen. Prementum carries ligula with 3-segments palp. Palpomere 3 longer than palpomere 1 and 2. Ligula diamond-shaped and sclerotized. Musculature: in Fig 8: pupa in B.
Adults(Fig 4: G-H). Elemental composition stays same to Day 11 pharate adult. Submentum edge not clear, recognized by muscles attachment. Labium possess well developed 3-segments palps. Ligula large and sclerotized with paired paraglossae. Musculature: in Fig 8: adult in B.
Third instar larvae. Epipharynx, ventral surface of anterior clypeolabrum, sclerotized and slightly convex. Posterior membranous epipharynx fused to anterior pharynx and posterior hypopharynx laterally, forming the dorsal part of the closed prepharygeal tube. Musculature: in Fig 8: larva in C.
Pupae. Day 11: Pharate adult anterior epipharynx membranous, covering basal mandible. Posterior epipharynx and hypopharynx fused to anterior pharynx margin, forming anterior pharynx. Musculature: in Fig 8: pupa in C.
Adults. Same to pharate adult in Day 11. Musculature: in Fig 8: adult in C.
Hypopharynx and salivarium.
Third instar larvae. Anterior hypopharynx closely connected with anterior labium. Weak sclerotized above prementum and mentum. Posterior hypopharynx laterally fused to posterior epipharynx, forming the ventral prepharygeal tube. Salivarium absent. Musculature: in Fig 8: larva in C-D.
Pupae. Day 11: Pharate adult hypopharynx not fully developed, fused to ventral pharynx. Salivarium and salivary duct not well-developed. Musculature: in Fig 8: pupa in C.
Adults. Hypopharynx forms a structural and functional unit with anterior labium. Anterior part extends to ligula. Dorsolaterally, oral arms slender and run along hypopharynx. Hypopharyngeal suspensorial sclerites forms lateral short branch, closely connected with ventral ridge of prementum. Salivary duct broad and quardrangular in cross section above submentum and mentum. Musculature: in Fig 8: adult in C.
Third instar larvae (Fig 8: larva in C-D). Anterior precerebral pharynx V-shape. Following region approximately quadrangular in cross section with indistinct longitudinal folds for muscles attachment. Protocerebrum pharynx gradually narrow distally and irregular in cross section. Musculature: in Fig 8: larva in C-D.
Pupae (Fig 8: pupa in C-D). Day 11：Pharate adult pharynx narrow especially beneath brain. Precerebral pharynx slightly wide anteriorly. Cross section nearly oval. Pharynx wall thick and longitudinal folds indistinct. Postcerebral pharynx narrow. Musculature: in Fig 8: pupa in C-D.
Adults (Fig 8: adult in C-D). Anterior precerebral pharynx wide and nearly round in cross section. Pharynx wall thin and no distinct longitudinal folds. Postcerebral pharynx suddenly wide with thick wall. Longitudinal folds present. Musculature: in Fig 8: adult in C-D.
Transformation of muscles inside cocoon
The transformation of head muscles from Day 1 to Day 12 are illustrated in Figs 9-10. The histolysis and rebuilding of the skeleton and muscles happens in the prepupal stage, with these structures developing gradually in the following days until the adult stage. For example, the mandible muscles 0md1 and 0md3 are reconstructed in detail in Fig 11. At Day 1, muscles are compressed by the inner cuticle. By Day 2, the inner cuticle is strongly compressed, and most muscles have disintegrated. New skeletal structures begin to form. At Day 3, remaining muscle tissues disintegrate continuously. By Day 4, new muscle granules are present. At Day 5, muscle fibers are present. More and more muscle fibers and bundles present in the following days. By Day 12, almost all muscles present in bundle form.
Cephalic nervous system
The main elements of the central nervous system are the brain and the subesophageal ganglion. The latter is the first ganglion of ventral nerve cord. The two with the frontal ganglion are the main elements of the cephalic nervous system.
Cerebrum, suboesophageal complex, and frontal ganglion
Third instar larvae (Fig 12: larva). Size of brain and suboesophageal ganglion (sog) about 20% that of the entire head capsule. Brain composed of the protocerebrum, deutocerebrum, and tritocerebrum. Protocerebrum dumbbell-shaped and optical nerves extremely slender with very slightly round lobe. Two thin antennal nerves originate from slightly protruding region of deutocerebrum. Frontal connectives originate from tritocerebrum and circumoesophageal connectives continuous with tritocerebrum. Sog ovoid-shaped below pharynx. All slender nerves of labium, maxilla, and mandible originate from sog. Frontal ganglion triangular, connecting with the protocerebrum and tritocerebrum by three curved frontal connectives.
Pupae (Fig 12: pupa). Day 11: Pharate adult volume of brain and suboesophageal complex small, occupying about 12.5% that of head capsule. Protocerebrum unrepresentative of dumbbell-shape. Optical nerves cylindrical with slightly round lobe. Antennal nerves slender and bending upwards. Tritocerebrum bears circumoesophageal connectives. Suboesophageal complex nearly oval. Frontal ganglion triangular and connected by two curved frontal connectives.
Adults (Fig 12: adult). Volume of brain and suboesophageal complex occupies about 33.3% that of head capsule. Protocerebrum dumbbell-shaped with two large optic neuropils. Suboesophageal complex oval. Triangular frontal ganglion connected by three nerves like larvae.
Transformation of brains inside cocoon
Transformation of brains from Day 1 to Day 12 is illustrated in Figs 9-10, 13. The larval brain does not disappear completely, but there is a morphological change. At Day 1, the brain becomes small and simple. Antennal nerves, optical neuropils, and mouthparts nerves distinctly short. Frontal ganglion disintegrated. By Day 2, brain strongly compressed and suboesophageal ganglion separated from brain due to disappearance of circumoesophageal connectives. From Day 3, brain stops compression but becomes more and more larger over following days. By Day 9, slender antennal nerves present. By Day 11, frontal ganglion present.