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OTHER TYPES OF LINEAR NESTS
All vespid wasps, most pompilid and sphecid wasps, and a few bees constructed nests such as described above with the partitions and closing plugs made of mud or agglutinated sand. However, some bees made different kinds of nests, also composed of cells in a linear series.
In the colletid bee Hylaeus the mother secretes a salivary substance which dries to form an extremely thin and delicate but impermeable transparent membrane over the inner walls of the cell. She fills this with a rather liquid pollen-nectar mix regurgitated from her crop, lays an egg on the outer surface of the mass, and then makes a transverse septum of the same diaphanous material. After constructing and storing a series of such cells, she leaves an empty vestibular cell and then seals the boring entrance with another partition of the salivary secretion.
In some megachilid bees the cell walls as well as the partitions between the cells and the closing plugs are made of vegetable material. The leaf-cutter bees of the genus Megachile (figs. 98-100) line the cell with a cup-shaped structure which they make by putting a number of rectangular leaf-cuttings, one within the other. A store of pollen and nectar is placed in the inner two-thirds of the cell, an egg is laid on the outer surface of this mass, and the cell is sealed by a number of circular leaf cuttings fitted down into the cell formed by the rectangular pieces. More rectangular cuttings are brought in to form the end and walls of the second cell, and so on, until there may be a series of a dozen cells in the boring. These species of Megachile do not make a vestibular cell as such, but they construct an equivalent by placing a number of very loosely fitted circular cuttings in the outer section of the boring.
The other genus of megachilid bees belonging to this category is Anthidium whose members are known as carder bees because they use cottony plant materials in the nest construction (figs. 79, 80). Anthidium maculosum lines the inner end of the boring and walls of the first cell with this matted cottony substance which it obtains from leaves of plants like cotton-wood (Populus) or desert willow (Chilopsis). Then the female stores a rather liquid nectar-pollen mixture in the cell, lays an egg on the outer surface of the mass, makes a closing partition of the cottony material, and continues to store a series of cells in the same way. She does not construct a vestibular cell; instead, she makes a very thick plug consisting of a section of wadded cottony fibers, which is sometimes followed by a middle section of diverse materials such as small pebbles, and then an outer section of more wadded cotton.
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The wasps and bees which use these traps exhibit the usual holometabolous development of egg-larva-pupa-adult. Most of them are multivoltine; that is, there are two or more generations a year. In nests of these species the occupants of overwintering nests pass the winter as resting or diapausing larvae, transform to pupae the following spring, and then to adults.
However, some species have only a single generation a year. The most abundant of these are the vernal bees of the genus Osmia. Occupants of their nests do not overwinter as resting larvae, but they transform to pupae the preceding summer and then to adults, which remain within the cocoons and do not chew their way out until the next spring. These Osmia bees are parasitized by species of the cuckoo wasp genus Chrysura and of the sapygid wasp Sapyga. These parasites have synchronized their development perfectly with that of their host bees. They transform to pupae and then to adults, concurrently with bee hosts, overwinter in their cocoons, and emerge concurrently with the bees the following spring.
A few species of vespid wasps are univoltine, having only a single generation, but the occupants of these nests overwinter as resting larvae and transform to pupae and adults late the following spring. In other words, no strictly vernal wasps nested in these borings.
When the mature larva finishes feeding, the body is plump and distended and has a more or less circular cross-section; the constrictions between the body segments are very weak (figs. 18, 21). The integument is transparent and glistening, and the tracheation, and frequently the oenocytes, may be seen through it. After the larva voids the accumulated fecal wastes and spins its cocoon, the body has a noticeably different appearance. The body always becomes somewhat flattened, the constrictions between the segments become very pronounced (cf. figs. 27, 28), the anterior end is usually bent downward and backward (fig. 20) although not so in Monobia (fig. 28) and Euodynerus (figs. 40, 41), and the integument becomes dull, opaque, and frequently finely wrinkled (fig. 19). The larva is usually quite flaccid, although in Monobia and some Euodynerus it is firm and leathery. This postdefecating larval stage, which is not preceded by a molt, is frequently called the prepupa. I have used the term prepupa in this report for the short period intervening between cocoon-spinning and pupation in larvae of the summer generation nests.
In nests of the overwintering generation (except in Osmia and its parasites), these postdefecating larvae enter a period of prolonged diapause during which development is inhibited. This diapause is broken by a period of exposure to chilly weather after which development is resumed. I have applied the terms diapausing or resting to larvae in these overwintering nests.
As the pupa develops within the larval integument, the body again becomes somewhat distended and the head end straightens out in those genera in which it was originally curved downward and backward. The darker posterior margin of the compound eye of the pupa is visible a day or two before the larval exuvia is shed.
The pupal stage is of variable duration depending upon the species. Figures 30-36 illustrate development of adult coloration within the pupal exuvia. Eclosion of the adult from this pupal cuticle occurs several days after the body becomes fully colored. After eclosion the adult remains in the cell for several days while the wings and integument harden.
Vespid nests are unique, differing from those of all other families of wasps and bees nesting in these borings in that the egg is laid in the inner end of the cell before any prey is stored in that cell. The other wasps and bees nesting in these borings first bring into the cell a store of food, paralyzed arthropods of various kinds for the wasps and a pollen-nectar mixture for the bees, before an egg is laid and the cell is sealed.
The deposition of the vespid egg is also unique among the wasps and bees in that it is suspended from a flexible thread several millimeters in length at a point several millimeters from the inner end of the cell (fig. 24). The eggs of other wasps are glued—loosely or firmly—to one of the individuals of prey stored for the waspling (e.g., figs. 43, 52, 62), whereas those of bees are usually partly embedded in the pollen-nectar mixture (figs. 87, 91).
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A question which intrigues many people is how the wasps in the inner cells are able to get out of the nest. They develop from eggs which are laid earlier and food must be provided before the cells in the outer end can be built and stored; sometimes several days elapse between provisioning the first and last cells in a nest. So one would imagine that the occupant of cell 1 should mature first and that the occupant of the last cell should mature last.
Several factors prevent this order of development. First, larval growth in a mixed vespid nest is a little slower in the inner, longer female cells where more food is stored. Second, males tend to pupate in 1-2 days less than females after completion of larval feeding. Third, the pupal period is 2-4 days shorter for male wasps. These different factors insure that in mixed vespid nests of the summer generation the males in the outer cells are ready to emerge several days earlier than their sisters in the inner cells.
Newly eclosed adults usually spend 2-4 days in their cells in order that the wings and integument can harden before they emerge from the nest. This situation does not alter the fact that males emerge before females, but it does operate to insure that the males in an individual nest frequently emerge concurrently or within a day of each other, and the same is true for the females several days later.
There are probably two signals that precipitate emergence of a series of males from an individual nest. When the day of emergence has arrived for the adult male in the outermost cell, he starts to chew through the partition capping his cell. He secretes salivary fluid which helps to moisten the partition sealing the cell and to make egress easier. The vibrations from his chewing are probably sensed by the wasp in the preceding cell who commences to gnaw at the partition capping his cell. His behavior is communicated to the wasp in the antepenultimate cell, and so forth, until each male is engaged in making a hole through his own cell partition. But occasionally one of the males, in the antepenultimate cell for example, decides it is time to emerge before he gets such a signal. When he starts to gnaw through the partition capping his cell, he would initiate similar action by the wasp in the penultimate cell and thence by the wasp in the outermost cell. I have actually seen this second kind of emergence beginning in nests which have been opened at just the appropriate time.
Occasionally it happens that the occupant of a cell near the boring entrance may die in the cell, either as a prepupa, pupa, or eclosed adult. This death requires the wasp in the cell immediately preceding that one to chew through an extra partition or two to emerge. Such effort is not always physically possible, so that the death of an individual in an outer cell may doom his or her predecessors in the earlier cells to a similar fate.
The discussion thus far has been concerned with emergence from nests of the summer generation. The occupants of overwintering vespid nests are all diapausing larvae. Using Euodynerus foraminatus apopkensis as an example, we find that when diapause is broken the males pupate 5 days before the females and have a shorter pupal period by 2 days. Consequently, their emergence from the nest takes place about a week before that of the females.
From overwintering nests emergence of various species in the laboratory followed a definite sequence corresponding to the first appearance of those species in nature. The earliest emergents were the vernal species such as the several species of Osmia and their parasites Chrysura kyrae, C. pacifica, and Sapyga centrata. Following these were several species of Ancistrocerus and Trypoxylon, Symmorphus cristatus and S. albo-marginatus, and Passaloecus cuspidatus. In the next week or two many species of Euodynerus and Stenodynerus, Symmorphus canadensis, and Isodontia emerged. The species of Trypargilum and Podium emerged later, after most of the other wasps.
Another interesting discovery during this study was the demonstration that individuals of the same species emerged at the same time in Washington even though some nests came from western New York, others from the metropolitan area of Washington, D. C, and still others from coastal North Carolina. For example, in nests of Trypargilum collinum rubrocinctum held outdoors in Washington over the winter of 1957-58, adults emerged on May 15-16 from nests stored at Derby, May 16-24 from nests stored at Plummers Island, and May 17-22 from nests stored at Kill Devil Hills. Had these nests been kept in the localities in which they were stored, we could have expected that the North Carolina population would have emerged 1-2 weeks earlier than the population around Washington, D. C, and 3-4 weeks earlier than the population in western New York.
Two other aspects of adult emergence merit some discussion. I have termed these divided emergence and delayed emergence.
Divided emergence occurred when one or several individuals emerged the same summer that nests were constructed, about a month afterward, whereas the other individuals in the same nest overwintered as diapausing larvae, which next spring transformed to pupae and then to adults. This behavior occurred chiefly in Trypargilum collinum rubrocinctum; males and some females from the outermost cells emerged the preceding summer, and the remaining females from the inner cells the following spring. This type of emergence also occurred in one nest of T. clavatum, in two nests each of T. t. tridentatum and T. t. archboldi, and one nest each of Euodynerus p. pratensis, E. schwarzi, Stenodynerus lineatifrons, and Ancistrocerus c. catskill.
Delayed emergence occurred when all but one or two occupants of an overwintering nest emerged as adults that spring, and one or two larvae continued in diapause all through a second winter and adults did not emerge until the next spring or summer. This phenomenon occurred most frequently, though still rarely, in 9 nests of Trypargilum striatum. The wasps which underwent this delayed emergence occurred at random in the nests and usually they died as fully colored pupae or newly eclosed adults in their cocoons. This delayed emergence also occurred in one nest of Ancistrocerus t. tuberculiceps, in my only nest of Podium luctuosum, and in six nests of the megachilid bee Prochelostoma philadelphi. The occupants undergoing delayed development in nests of the two wasps usually did not transform to viable adults. However, the occupants undergoing delayed development in the bee nests emerged at the proper season 2 years after their nests were constructed. In the bees this is probably a useful adaptation to insure that part of the progeny will be available for a second season in case the earlier emergents do not find good pollen and nectar sources the previous spring.
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Posted October 30, 2009