Frederick W. Stehr

Ocelli and Stemmata

Publisher Summary This chapter discusses the types of simple eyes in insects namely ocelli and stemmata. Until relatively recently, the simple eyes of both larvae and adults have been termed “ocelli” (ocellus), although it has been recognized that these are two different groups that are innervated from different parts of the brain. One group of simple eyes, found in adult insects and larvae of nonholometabolans, is termed “dorsal ocelli” or simply “ocelli.” These structures are innervated dorsally from the protocerebrum between the optic lobes. The second group of simple eyes, formerly termed “lateral ocelli” but now termed “stemmata” (singular stemma), is found in the larvae of Holometabola. Stemmata are innervated laterally from the optic lobes, and typically there is a group on each side of the head. The number of stemmata is variable, ranging from zero to seven, and the number and arrangement can be diagnostic.

Pupa and Puparium

Publisher Summary This chapter describes the pupa stage in insect life cycle. Pupa is the stage in the development of holometabolous insects between the mature larva and the adult wherein major morphological reorganization takes place. Pupation usually occurs in a protected location (in a cell or cocoon), but in some groups, such as many butterflies, the pupa (chrysalis) is suspended openly and is usually well camouflaged by its shape and color. There are two basic kinds of pupae, exarate and obtect. An exarate pupa has free appendages. An obtect pupa has the appendages adhering to the body wall. Most Lepidoptera, most lower Diptera, some chrysomelid and staphylinid beetles, and many chalcidoid Hymenoptera have obtect pupae; nearly all other pupae are exarate. Most pupae are inactive, their body movements often limited to the abdominal segments. However, pupae in some groups are capable of locomotion, and some have functional mandibles that enable them to cut their way out of the pupal cell, cocoon, or chamber. These active pupae are sometimes referred to as pharate adults because the adult is enclosed in the pupal cuticle.

Chapter 148 – Larva

Publisher Summary This chapter focuses on the larval stage of insect lifecycle. The term “larva” is used for all immatures that are not eggs, pupae, or adults. When larva is used in this comprehensive sense, the subcategories include exopterygote larva, for Hemimetabola, which have the wingpads developing externally, and endopterygote larva, for Holometabola that have the wings developing internally as histoblasts in the larva. Larvae occur in a great diversity of sizes, shapes, and colors. Colorful ones almost always live in exposed habitats where their colors and shapes offer cryptic concealment or where their bright colors and spines warn potential predators that they are not to be eaten. Larvae that live in concealed habitats are nearly always combinations of white, gray, black, or brown. There are some general terms used for types of holometabolous larvae that have broad usage such as campodeiform larvae, elateriform larvae, scarabaeiform larvae, eruciform larvae, and vermiform larvae. Campodeiform larvae are somewhat flattened and have an elongate body, thoracic legs that are well developed, a head that is directed forward, no abdominal prolegs, and antennae and cerci that are usually conspicuous. Elateriform larvae is similar to campodeiform larvae, but their body is more elongated, subcylindrical, and more heavily sclerotized. Scarabaeiform larvae have a C-shaped, whitish body with a dark head, and well-developed thoracic legs. Eruciform larvae are caterpillar-like and have a cylindrical body and well-developed thoracic legs, and prolegs are present. Finally, vermiform larvae are “wormlike.” It is generally applied to an elongate, legless larva with or without a conspicuous head.

Chapter 166 – Metamorphosis

Publisher Summary This chapter discusses the metamorphosis in insects. Most organisms undergo a change in form as they grow from an embryo to an adult. The term larva has very broad usage in invertebrate zoology, being applied to an assortment of forms (often the dispersive state) in virtually all invertebrate phyla. In the insects, larva has been used in different ways, including such diverse forms as the immatures of the most primitive order Protura and of the most advanced order Hymenoptera (sawflies, ants, wasps, and bees). The termites present an interesting problem: some authors use nymph for all juvenile termites, whereas others use larva for those lacking wingpads, and nymph for those having wingpads. This is further complicated because the supplementary reproductives may be wingless or bear wingpads, even though the two forms are functionally equivalent. The kind of metamorphosis insects undergo is closely related to which of the subclasses a taxon belongs to. In the Apterygota, metamorphosis is either anamorphic in the Protura, in which three abdominal segments are added as the individual develops to an adult, or it is ametabolous, in which the number of molts is indefinite and molting may continue throughout life after sexual maturity. In the Pterygota, there are two fundamental kinds of metamorphosis: the hemimetabola develop through the egg, larva, and adult stages, and the holometabola develop through the egg, larva, pupa, and adult stages. There are also other terms that have been used to describe variations in metamorphosis.

Chapter 49 – Cocoon

Publisher Summary This chapter discusses cocoon, which is a silk-protective covering within which the caterpillars of many moths and a few butterflies pupate. Other orders of insects also spin silk and form cocoons, including Siphonaptera (fleas), Hymenoptera (ants, bees, and wasps), Neuroptera (lacewings and antlions), and Trichoptera (caddisflies). Lepidoptera cocoons can be very thick and tough, such as that of the silkworm, Bombyx mori, or they can be limited to a relatively few strands of silk that keep the pupa from falling or that hold materials together to form a shelter. There are many kinds of lepidopteran cocoons. Some are formed from substrate materials held together by silk, others are so tough and jug-like that they have a special escape lid woven at the end for the emergence of the adult lepidopterans such as the cecropia moth, Hyalophora cecropia (Saturniidae), spin an elongate cocoon with a one-way escape hatch at the end. Hymenopteran cocoons are highly variable in appearance, ranging from the tightly spun cocoons of some parasitic ichneumonid wasps that are suspended on a long silk thread to the more loosely spun cocoons of some braconid wasps such as those found clustered on the outside of caterpillars. Flea (Siphonaptera) larvae spin cocoons covered with debris in areas where the larvae have been feeding. Lacewing larvae (Neuroptera: Chrysopidae) spin tight, egg-shaped cocoons in some snug spot near where they have been feeding.

Chapter 45 – Chrysalis

Publisher Summary This chapter discusses chrysalis, which is the pupa of a butterfly, usually belonging to the family Papilionidae, Pieridae, or Nymphalidae. In forming the chrysalis, the prepupal caterpillar has to perform the seemingly impossible maneuver of spinning the silk pad to attach its cremaster (caudal pupal hooks) while maintaining its grip; it then must molt the larval skin as it attaches to the silk pad. Members of the Pieridae and Papilionidae (except Parnassinae) secure the chrysalis in an upright position with a band of silk around the middle. Chrysalids are usually angular, with projections, tubercles, spines, and sometimes gold or silver flecks. They are often cryptically colored so that they blend into the surrounding materials but some, such as the monarch chrysalis, are smooth with gold flecks. In emerging from the chrysalis, the adult splits the chrysalis ventrally and dorsally at the anterior end, crawls out, and suspends itself from the pupal skin while its wings expand.

Chapter 36 – Caterpillars

Publisher Summary This chapter discusses caterpillars. The larvae of butterflies, skippers, and moths of the order Lepidoptera are generally known as caterpillars. Caterpillars come in a diversity of sizes, shapes, and colors. The most common form has a conspicuous head, a thorax with three pairs of legs, and an abdomen with five pairs of prolegs that bear crochets (hooks) that enable the caterpillar to cling tightly to or wedge itself between materials. Caterpillars are commonly encountered because many are leaf feeders and are not concealed feeders, although they may be cryptically shaped or colored. The realistic twig mimicry and behavior found in some inchworms (Geometridae) are complete with body markings that resemble bark irregularities, scars, and stipules. Another excellent mimic is the caterpillar of the orangedog, Papilio cresphontes, whose black and white larvae resemble bird droppings in all instars. Most caterpillars move by a wavelike movement of the legs from rear to front. However, those with reduced numbers of prolegs or none at all proceed in a different manner. Caterpillars may be aposematically colored to advertise that they are distasteful or poisonous. They may also bear diverse lobes, spines, horns, knobs, and urticating hairs or spines that may or may not be irritating in some way.