These larvae need to be fed both nectar for carbohydrates and pollen for protein in order to develop. Bumblebees feed larvae nectar by chewing a small hole in the brood cell into which nectar is regurgitated. Larvae are fed pollen in two ways, depending on the bumblebee species. So called "pocket-maker" bumblebees create pockets of pollen at the base of the brood cell clump from which the larvae can feed themselves. Conversely, "pollen-storer" store pollen in separate wax pots and feed it to the larva in the same fashion as nectar. Bumble bees are incapable of trophallaxis, direct transfer of food from one bee to another. With proper care, the larvae progress through four instars, becoming successively larger with each molt. At the end of the fourth instar the larvae spin silk cocoons under the wax covering the brood cells, changing them into pupal cells. The larvae then undergo an intense period of cellular growth and differentiation and become pupae. These pupae then develop into adult bees, who chew their way out of the silk cocoon. When adult bumble bees first emerge from their cocoons, the hairs on their body are not yet fully pigmented and are a greyish-white colour. The bees are referred to as "callow" during this time, and they will not leave the colony for at least 24 hours. The entire process from egg to adult bee can take as long as five weeks, depending on the species and the environmental conditions. After the emergence of the first or second group of workers, workers take over the task of foraging and the queen spends most of her time laying eggs and caring for larvae. The colony grows progressively larger and at some point will begin to produce males and new queens. The point at which this occurs varies among species and is heavily dependent on resource availability and environmental factors. Unlike the workers of more advanced social insects, bumble bee workers are not physically reproductively sterile and are able to lay haploid eggs that develop into viable male bumble bees. Only fertilized queens can lay diploid eggs that mature into workers and new queens. Early in the colony cycle, the queen bumble bee compensates for potential reproductive competition from workers by suppressing their egg-laying by way of physical aggression and pheromonal signals. Thus, the queen will usually be the mother of all of the first males laid. Workers eventually begin to lay males later in the season when the queen's ability to suppress their reproduction diminishes. The reproductive competition between workers and the queen is one reason that bumble bees are considered "primitively eusocial". New queens and males leave the colony after maturation. Males in particular are forcibly driven out by the workers. Away from the colony, the new queens and males live off nectar and pollen and spend the night on flowers or in holes. The queens are eventually mated, often more than once and search a for suitable location for diapause. Bumblebees generally visit flowers exhibiting the bee pollination syndrome. They can visit patches of flowers up to 1-2 kilometres from their colony. Bumblebees will also tend to visit the same patches of flowers every day, as long as nectar and pollen continue to be available. While foraging, bumblebees can reach ground speeds of up to 15 m/s (54 km/h). When bumblebees arrive at a flower, they extract nectar using their long tongue "glossa" and store it in their crop. Many species of bumblebee also exhibit what is known as "nectar robbing": instead of inserting the mouthparts into the flower normally, these bees bite directly through the base of the corolla to extract nectar, avoiding pollen transfer. These bees obtain pollen from other species of flowers that they "legitimately" visit. Pollen is removed from flowers deliberately or incidentally by bumblebees. Incidental removal occurs when bumblebees come in contact with the anthers of a flower while collecting nectar. The bumblebee's body hairs receive a dusting of pollen from the anthers which is then groomed into the corbiculae "pollen baskets". Bumblebees are also capable of buzz pollination. In at least a few species, once a bumblebee has visited a flower, it leaves a scent mark on the flower. This scent mark deters visitation of the flower by other bumblebees until the scent degrades.. It has been shown that this scent mark is a general chemical bouquet that bumblebees leave behind in different locations (e.g. nest, neutral and food sites), and they learn to use this bouquet to identify both rewarding and unrewarding flowers. In addition, bumblebees rely on this chemical bouquet more when the flower has a high handling time i.e. it takes a longer time for the bee to find the necta). Once they have collected nectar and pollen, bumblebees return to the nest and deposit the harvested nectar and pollen into brood cells, or into wax cells for storage. Unlike honey bees, bumblebees only store a few days' worth of food and so are much more vulnerable to food shortages. However, because bumblebees are much more opportunistic feeders than honey bees, these shortages may have less profound effects. Nectar is stored essentially in the form it was collected, rather than being processed into honey as is done in honey bees; it is therefore very dilute and watery, and is rarely consumed by human. Bumblebees of the subgenus Psithyrus (known as cuckoo bumblebees, and formerly considered a separate genus) are a lineage which has lost the ability to collect pollen, and live parasitically in the colonies of other bumblebees. Before finding and invading a host colony, a Psithyrus female (there is no caste system in these species) will feed directly from flowers. Once she has infiltrated a host colony, the Psithyrus female will kill or subdue the queen of that colony and forcibly (using pheromones and/or physical attacks) "enslave" the workers of that colony to feed her and her young. The female Psithyrus also has a number of morphological adaptations, such as larger mandibles and a larger venom sac that increase her chances of taking over a nest. Upon hatching, the male and female Psithyrus disperse and mate. Like non-parasitic bumblebee queens, female Psithyrus find suitable locations to spend the winter and enter diapause upon being mated.