[Definitions and senses are in historical order]

doodlebug noun [Americanism (U.S.), from doodle + bug]

1. the larva of certain antlions; broadly: any of several other insects
2. one who does not stand up for his convictions
3. [colloquial senses]
   1. [sensing device or user of such device]
      
      1. an unscientific device (as a divining rod) for locating underground gas, water, oil, or ores
      2. one professing skill with such a contrivance
      3. a seismograph, gravimeter, or other scientific device for the location of minerals
   2. a small tractor used in lumbering or farming
   3. [railcar]
      
      1. a rail motorcar used by railroad employees in construction and maintenance-of-way work
      2. a small local train
   4. [certain military vehicles]
      
      1. a small military tank
      2. an army utility truck
   5. robot bomb
   6. a magnetic detecting device; specifically: an airborne magnetometer used for spotting submarines
   7. a midget racing automobile
   8. a very small airplane

doodle

1. doodle noun -s [perhaps from Low German dudeldopp]
   a foolish or frivolous person
2. doodle verb, transitive -ed/-ing/-s
   dialect :to make a fool of; cheat
3. doodle verb, transitive -ed/-ing/-s [German dudeln, to play (the bagpipe), from dudel, bagpipe, from Czech or Polish dudy, bagpipe, from Turkish duduk a flute; akin to Russian dudá fife, shawm, Lithuanian daudytè shawm]
   [British dialect] : to play on (the bagpipe)
4. doodle noun -s [short for haydoodle, perhaps from hay + doodle (in cock-a-doodle-do), euphemism for -cock (associated with cock penis)]
   [Dialect] : a small pile of hay in the field; haycock
5. doodle noun -s [by shortening]
   doodlebug
6. doodle verb doodled; doodled; doodling; doodles [Americanism, perhaps from doodle (2) and (3)]
   verb, intransitive 1: to make a doodle 2: to engage in aimless, haphazard, or inconsequential activity : dawdle, trifle, toy 3: [Colloquial] to play music in a casual, informal way
   verb, transitive 1: to mark or overspread with doodles 2: to expend in doodling 3: to trace in the manner of a doodle
7. doodle noun -s [Americanism, from doodle (6)]
   an aimless more or less automatic scribble, outline, design, or improvised sketch traced while one is mentally occupied with something else; doodler noun

bug

1. bug noun -s [Middle English bugge scarecrow; akin to German dialect bögge piece of dried nasal mucus, hobgoblin, Norwegian dialect bugge big or important man]
   obsolete : bogey, bugbear; hobgoblin
2. bug noun -s [origin unknown but probably from previous]
   
   1. [animal]
      1. an insect or other creeping or crawling invertebrate (as a spider or small crustacean)—not used technically
      2. any of certain insects commonly considered especially obnoxious, such as:
         1. bedbug
         2. cockroach
         3. head louse
      3. an insect of the order Hemiptera; especially: a member of the suborder Heteroptera
   2. an unexpected defect, fault, flaw, or imperfection
   3. [colloquial senses]
      1. a disease-producing germ or other microorganism
      2. a disease caused by such bugs; especially: any of various respiratory conditions of virus origins (as influenza or grippe)
   4. [American slang]
      1. fad, craze, hobby
      2. enthusiasm, concern, or deep interest esp. in respect to some particular matter or objective
      3. a person notably concerned with, enthusiastic about, or efficient at a specified interst or activity
      4. hobbyiest
      5. a crazy person; especially : firebug
   5. [person]
      1. [Archaic] a vain or self-important person
      2. a person of prominence or high social standing
   6. [poker] the joker when considered wild only for the purpose of filling straights or flushes or of acting as an ace
   7. [American slang]
      1. an alarm system (as a burglar alarm)
      2. a concealed microphone
      3. a device for wiretapping
      4. a high-speed telegrapher's key that makes repeated dots or dashes automatically and saves motion of the operator's hand
   8. [so called from its designation by an asterisk on race programs] the weight allowance (5 pounds, or 2.3 kg) given apprentice jockeys for one year after the riding of five winners
   9. [Slang] numbers game
   10. [American slang] a light usually two-seater stripped-down automobile
   11. a fishing plug thought to resemble a large insect