The Cuckoo Bumblebees are species of bumblebees that belong to the taxonomic subgenus Psithyrus. They are well known for their parasitic lifestyle involving the takeover of other other bumblebee nests. Cuckoos are usual social insects that are recognized by the black and yellow/orange body hairs, often in rings around its abdomen. They commonly have skinnier legs due to them not being able to hold and obtain honey sacks. The concern of extinction is growing and is being monitored. Recently they have been raised and used for pollination in green houses. 
Most recognizable, the Cuckoo Bumblebee is similar to furry yellow bees. Cuckoos are usual social insects that are recognizable with black and yellow body hairs, forming different patterns around the body. They commonly have skinnier legs due to not being able to hold and obtain honey sacks. Bumblebees range in size from about half an inch to a full inch in length with usual patterns. Even though the size, color and patter can distinguish species, they can still vary in qualities quite a bit. Cuckoo bumblebees resemble other bumblebees but lack the ability to gather pollen, forcing them into a parasitic lifestyle. The bumblebee's body is divided into three main parts: The head, which can be quite difficult to see, the thorax which is the biggest being the flight muscle, that has the wings and legs attached with various large black rings, and the abdomen which has the sting, the wax glands and all the digestive and reproductive organs.  
In the Autumn, young cuckoo queens mate with males and diapause (mating/fraternal period) during the winter. Commonly they retreat to a sheltered area during this process weather it be in the ground or in a man-made structure. In early spring, the queen comes out of isolation and finds a suitable place to create her colony. the Cuckoo Bumblebee, taking on a parasitic lifestyle, establishes and locates a nest by smell. When she has found a nest that is pleasing to her she will go in and sting the existing queen to death, taking over the hive and using the workers to rear her young. Another procedure that is common, is to sneak in and hide for a couple days, absorbing the scent of the hive. When she has finally obtained the natural smell of the hive, she would be accepted and would feel comfortable laying her eggs there. This method is destructive for the nest because Cuckoo larva only consumes resources and do not contribute. Soon the the hive will deteriorate and the queen bee will go and find another male and repeat this process.  
Cuckoo Bumblebees are usually found in higher elevation, but exceptions are currently becoming more common. Specific species vary in very cold climates where other bees are not usually found. One reason for this is that Cuckoos (as other bees) can regulate their body temperature, radiation, internal mechanisms of shivering and radiative cooling from abdomen. Other bees have similar physiology but are not as noticeable. 
Bumblebees are in danger in many countries because of the habitat and the destruction to nature's natural life habitat. In Britain, 19 species of native bumblebees were discovered along with cuckoo bumblebees to have become extinct, eight are in serious decline, and only six remain widespread. A decrease in bumblebee population could cause drastic changes to the countryside, leading to inadequate pollination of certain plants.
Now a Days
Now a days, bumblebees are being raised for agricultural use as pollinators because they can pollinate plant species that other pollinators cannot by using a technique known as "buzz pollination". For example, bumblebee colonies are often placed in tomato greenhouses, because the frequency of buzzing from a bumblebee effectively releases tomato pollen.  
One misconception of bees is the assumption that the buzzing noises from a bumble bee is that of their beating wings. When actually the sound is the bee vibrating its flight muscles which are separated from the wings—a feature only known in bees. This is especially emphasized in bumblebees, as they warm up their bodies to get airborne and decrease in temperature. Bumblebees have been known to reach an internal temperature of 30 °C (86 °F) using this method.
The familiar buzz of a flying Bumblebee inspired the famous orchestral piece "Flight of the Bumblebee". 
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- Bumblebees - the Genus Bombus by About.com, New York Times Comp.
- Bee by Wikipedia, 20 November 2009